Review of Cabling Techniques and Environmental Effects Applicable

REVIEW OF CABLING TECHNIQUES 

AND ENVIRONMENTAL EFFECTS 

APPLICABLE TO THE OFFSHORE 

WIND FARM INDUSTRY 

Technical Report 

JANUARY 2008 

IN ASSOCIATION WITH

Contents 

1 Introduction 11 

1.1 Background 11 

1.2 Study Description 12 

1.3 Report Format 13 

2 Legislation 15 

2.1 Introduction 15 

2.2 Licences and Consents 15 

3 Cable types and installation techniques 18 


3.2 Cable Types 18 

3.3 Offshore Wind Farm Cables 18 

3.4 Telecoms Cables 22 

3.5 Power Cables 26 

3.6 Flowlines, Umbilicals and Small Diameter Pipelines 27 

3.7 Background to Safe Installation and Protection of Subsea Cables 27 

3.8 Cable Protection Methods 34 

3.9 Cable Protection for Offshore Wind Farm Developments 70 

3.10 Burial Assessment and General Survey Techniques 77 

3.11 Decommissioning 78 

4 Physical change 80 


4.2 Site Conditions 80 

4.3 Level of Sediment Disturbance 82 

4.4 Seabed Disturbance by Other Activities 89 

4.5 Dispersal and Re-Deposition of Sediment 90 

4.6 Mitigation Measures 99 

5 Potential impacts and mitigation measures 101 


5.2 Subtidal Ecology 102 

5.3 Intertidal Habitats 110 

5.4 Natural Fish Resource 113 

5.5 Commercial Fisheries 121 

5.6 Marine Mammals 125 

5.7 Ornithology 128 

5.8 Shipping and Navigation 131 

5.9 Seascape and Visual Character 133 

5.10 Marine and Coastal Archaeology 134 

6 Good practice measures 138 

7 Gaps in understanding 140 

8 References 142 

Appendix A: Standards and codes of practice relevant to 

cable installation & the Offshore wind industry 154 

1

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind 

Farm Industry – Technical Report 

Acknowledgements 

This report was prepared by consultants from Royal Haskoning and BOMEL 

Ltd. The principal contributors from Royal Haskoning were Dr Samantha Vize, 

Christine Adnitt, Robert Staniland, Julia Everard, Alec Sleigh, Rod Cappell, Sean 

McNulty and Dr Martin Budd. From BOMEL Ltd, the principal contributors were 

Ian Bonnon and John Carey. 

This report has benefited from assistance from Adrian Judd at the Centre of 

Fisheries, Environment and Aquaculture Science (CEFAS). The authors also wish 

to thank the following consultees: 

Natural England (Louise Burton, Victoria Copley, Keith Henson, Sarah Soffe and 

Michael Young) 

English Heritage (Dr Chris Pater) 

Countryside Council for Wales (Dr Sarah Wood) 

Joint Nature Conservation Committee (Zoe Crutchfield and Andrew Prior) 

These consultees have assisted in providing comments on the principles of 

the study via correspondence. Their citation here does not indicate current 

agreement or support of the study outputs. 

The research leading to the publication of this report was funded by: 

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2 

Department for Business, Enterprise and Regulatory Reform; and 

Department for Environment, Food and Rural Affairs. 

The project was managed by Dr John Hartley of Hartley Anderson Ltd on behalf 

of the Department Business, Enterprise and Regulatory Reform. 

The Target Audience 

This report is intended to provide technical details on subsea cable installation 

techniques and associated potential environmental effects, particularly relating 

to the offshore wind farm industry. It aims to inform wind farm developers (and 

their consultants), stakeholders and regulators during the Environmental Impact 

Assessment (EIA) stage and consents process.

Summary 

In the UK today, wind farms are the most developed technology for the 

large scale production of renewable energy offshore. Given their position 

away from land-based urban development, offshore wind farms can be more 

substantial in terms of their area and power generation, compared to their landbased 

counterparts. Public support for offshore wind farms is generally high, 

particularly where evidence is presented through the formal consenting and 

consultation processes that developments are sited in an appropriate location, 

where environmental and negative economic effects are minimal or can be 

effectively managed. 

Integral to offshore wind farm development is the installation, operation and 

maintenance of the supporting electrical infrastructure of intra-array and export 

cables. This document aims to provide an information resource, intended for 

use by wind farm developers, consultants and regulators, on the range of cable 

installation techniques available, their likely environmental effects and potential 

mitigation, drawing on wind farm and other marine industry practice and 

experience. Through the collation of existing information and experience from a 

range of sources, the report will assist government, developers, stakeholders and 

regulators during the formal Environmental Impact Assessment and consenting 

process including stages of information provision, review and approval of such 

information. Importantly, an understanding of the difficulties and constraints of 

cable installation has been provided such that impacts can be avoided, reduced 

or minimised. This document also deals with the practical application of the 

installation and mitigation techniques available to developers so that the most 

relevant and up-to-date technology can be applied in the most appropriate 

situation. 

This document covers: 

● 

● 

● 

The range of types of cables and small diameter pipelines currently installed 

in the EU shelf marine environment. Details are given for the types of cable 

typically used for offshore wind farms and also the types of cable commonly 

used in the telecommunications and power cable industries. 

The range of techniques used to install and maintain the aforementioned 

cables and pipelines. Information is provided on a range of commonly 

applied cable protection measures, and also of the burial assessment survey 

techniques, commonly used before cable installation. 

The physical changes or effects to the seabed and sub-surface sediments 

expected to occur during cabling activities. The range of sediment types likely 

to be encountered during cable burial operations is discussed, with the level 

of sediment disturbance that is likely to occur during cable burial for each 

technique. 

3



● 

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● 

4 

The potential environmental impacts that could occur during the installation 

and maintenance of subsea cables. Impacts are described and discussed 

for intertidal habitats, subtidal ecology, natural fish resources, commercial 

fisheries, marine mammals, ornithology, shipping and navigation, seascape 

and visual character, and marine and coastal archaeology. Impacts are 

divided into those that could cause potentially significant impacts and those 

that may arise but are unlikely to cause significant effects. 

Mitigation measures that can be used to reduce the level of significance of 

environmental effects. 

Examples of good practice measures that could be adopted during all phases 

of project planning and used in conjunction with mitigation measures to 

reduce potential disturbance from cabling activities; and 

Knowledge gaps identified during this review, including gaps in understanding 

of the actual environmental impacts resulting from cable burial activities. 

The key impacts relating to cable laying in the marine environment occur during 

the installation process. Cable burial and other protection measures (i.e. outer 

protective cable armouring, concrete and frond mattresses, rock dumping, and 

grout / sand bags) are used in order to reduce the risk of damage to the cable 

and to ensure the safety of other users of the sea. Where cables are not afforded 

adequate protection, damage can occur through abrasion (i.e. on rocky seabed 

types) and interaction with human activities such as snagging or entanglement 

from bottom trawl fishing gear, anchoring or navigational dredging. 

Dependant upon prevailing conditions of depth, seabed morphology, 

hydrodynamics and geology, a range of cable burial technology is available 

to developers, such as cable ploughs, tracked burial machines, free swimming 

remotely operated vehicles (ROVs) and burial sleds. 

Induced changes to the prevailing physical conditions of an area, such as the 

suspension, dispersal and subsequent deposition of seabed sediments, changes 

to seabed morphology and the direct impacts associated with the presence and 

operation of cable burial equipment can lead to a range of potential environmental 

impacts. The nature, extent and significance of these impacts will be a function 

of site specific characteristics (i.e. seabed type, tidal and wave conditions) and 

the chosen installation method. The key potential environmental impacts, which 

can, in specific circumstances, be associated with cable burial, include: 

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● 

● 

Disturbance to sessile, encrusting, and attached fauna and flora which can be 

dislodged/disturbed; 

Smothering of sessile species due to increased sediment deposition and sidecast; 

Damage to the filtering mechanisms of certain species, the gills of sensitive 

fish species and eggs and larvae from increases in suspended sediment and 

subsequent deposition;

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● 

● 

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The potential release of contaminants, previously retained in the sediment; 

Disturbance to important fisheries habitats such as spawning grounds, 

nursery grounds, feeding grounds, over-wintering areas for crustaceans, 

migration routes and shellfish beds; 

Disturbance to sensitive habitats, such as saltmarshes, biogenic reefs and eel 

grass beds; 

Noise and vibration impacts to fish and marine mammals; 

Electromagnetic field generation impacts to benthic species, fish and marine 

mammals; 

Risk of collision of marine mammals with cable installation vessels / support 

vessels; marine mammals may be disturbed by the presence of vessels and 

cable burial equipment; and, cable entanglement with marine mammals; 

Marine and coastal bird species, particularly at sites of importance to nature 

conservation, may be disturbed by the presence of vessels and human 

activities; 

Navigational risks such as collision with cable installation vessels; 

Direct loss or disturbance to artefacts of importance to marine and coastal 

archaeology and cultural heritage; and 

Seascape and visual impacts related to the cable installation activities and 

the presence of cable installation vessels; and possible sea surface aesthetic 

effects arising from sediment plumes, particularly in areas of chalk bedrock. 

This study concludes that, although cabling can cover large areas of seabed, the 

associated environmental impacts are highly transitory, localised in extent and 

temporary in duration. Although the corridor for cable installation impacts can 

be long, the footprint of impact is narrow, generally restricted to 2-3m width. For 

the majority of installation scenarios, the seabed and associated fauna and flora 

would be expected to return to a state similar to the pre-disturbance conditions. 

Exceptions could occur in hard clays and rock seabed types, where the cable 

trench would not naturally backfill, requiring intervention to backfill as part of 

construction works or else leaving permanent scarring of the seabed. 

The environmental impacts associated with the installation, operation and 

maintenance of cables associated with offshore wind farms must be considered 

in the context of other influencing factors. Natural perturbations such as storm 

activity can have a significant effect on the structure and functioning of the 

seabed, as can other activities such as oil and gas exploration and infrastructure, 

telecommunication cable installations, certain fishing activities, aggregate 

extraction, and other sources of change to the physical environment. In many 

cases, such influencing factors may lead to related environmental impacts of 

greater extent, duration and significance than those observed or suspected to 

arise as a result of the installation of offshore wind farm cable infrastructure. 

5

Abbreviations 

AIS Automatic Identification System 

AONB Area of Outstanding Natural Beauty 

AoSP Areas of Special Protection 

BAS Burial Assessment Survey 

BERR Department for Business, Enterprise and Regulatory Reform 

BETTA British Trading and Transmission Arrangements 

BWEA British Wind Energy Association 

BS British Standards 

BSC Balancing and Settlement Code 

CEFAS Centre for Environment, Fisheries and Aquaculture 

CIRIA Construction Industry Research and Information Association 

CIWEM Chartered Institute of Water and Environmental Management 

CMACS Centre for Marine and Coastal Studies 

COWRIE Collaborative Offshore Wind Research into the Environment 

CPA Coast Protection Act (1949) 

CRoW Countryside and Rights of Way Act (2000) 

CUSC Connection and Use of Systems Code 

DEFRA Department for Environment, Food and Rural Affairs 

DNO Distribution Network Operator 

DNV Det Norske Veritas 

DSV Dive Support Vessel 

DTI Department of Trade and Industry 

EA Electricity Act (1989) 

EHS Environment and Heritage Service (Northern Ireland) 

EIA Environmental Impact Assessment 

EMF Electro Magnetic Field 

ENA Electricity Networks Association 

ES Environmental Statement 

EU European Union 

FEPA Food and Environment Protection Act (1985) 

GCR Geological Conservation Review 

7



HAP Habitat Action Plan 

HSE Health & Safety Executive 

HVDC High Voltage Direct Current 

IEC International Electro-technical Commission 

IEEE Institute of Electrical and Electronics Engineers 

ISO International Organisation for Standardisation 

JNAPC Joint Nautical Archaeology Policy Committee 

JNCC Joint Nature Conservation Committee 

LWM Low Water Mark 

MCA Maritime and Coastguard Agency 

MCEU Marine Consents and Environment Unit 

MCGA Marine and Coastguard Agency 

MEHRA Marine Environmental High Risk Area 

MESH Mapped European Seabed Habitats 

MHWS Mean High Water Springs 

MLWS Mean Low Water Springs 

MNR Marine Nature Reserve 

MSPP Marine Spatial Planning Pilot 

NNR National Nature Reserve 

ORCU Offshore Renewables Consents Unit 

OREI Offshore Renewable Energy Installations 

PIANC Permanent International Association of Navigation Congresses 

REZ Renewable Energy Zone 

ROV Remotely Operated Vehicle 

RSPB Royal Society for the Protection of Birds 

SAC Special Area of Conservation 

SAP Species Action Plan 

SEA Strategic Environmental Assessment 

SMA Sensitive Marine Area 

SMP Shoreline Management Plan 

SPA Special Protection Area 

SSSI Site of Special Scientific Interest 

8

TTS Temporary Threshold Shift 

TWA Transport and Works Act 

UKBAP UK Biodiversity Action Plan 

UKOOA United Kingdom Offshore Operators Association 

VTS Vessel Traffic Services 

WAG Welsh Assembly Government 

WSI Written Scheme of Investigation 

WTG Wind Turbine Generator 

Abbreviations 

9

1 

Introduction 

1.1 Background 

The UK is committed to working towards a 60% reduction in CO 2 emissions by 

2050 and the development of renewable energy technologies, such as wind, is a 

core part of achieving this aim. As a carbon free source of energy, wind power 

contributes positively to the UK’s efforts to reduce our carbon emissions to tackle 

the threat of climate change (Sustainable Development Commission, 2005). The 

UK’s offshore wind energy resource is substantial, estimated at around 100,000 

GWh of practical resource (Sinden, 2004). Other marine renewables, such as 

wave and tidal are also rapidly developing sectors of the renewable energy 

industry. 

Whilst many of the impacts of offshore wind farm developments are becoming 

increasingly accepted and understood, concerns have been raised about the 

potential environmental effects of certain cable installation techniques (including 

by fishermen and country conservation agencies). There is a range of potential 

cable installation and protection technologies but to date, there has been limited 

centralised knowledge sharing both within the offshore wind industry and from 

other more established marine industries, such as subsea telecommunications 

and oil and gas. This lack of review and synthesis can hinder the consideration of 

Environmental Statements and adoption of best practice by developers involved 

in the area of marine renewables. 

The installation, operation and maintenance of the supporting electrical 

infrastructure of inter-array and export cables are integral to offshore wind farm 

development. A range of methods for cable installation are available to wind 

farm operators, the most applicable being dependant upon prevailing conditions 

of depth, seabed morphology, hydrodynamics and geology. These site specific 

characteristics and the chosen installation method subsequently manifest a 

range of impacts on the surrounding environment, the nature and extent of 

which can vary significantly. 

11



Figure 1.0 Scroby Sands offshore wind farm 

(Photo courtesy of Vestas Wind Systems A/S) 

1.2 Study Description 

A large number of cables associated with energy production, telecommunications 

and oil and gas extraction are already located on or under the seabed of the 

UK continental shelf. The effects of laying these cables have been investigated 

to varying degrees and, as such, a great deal of experience and previous 

knowledge has already been gathered and is held by various industries, 

government departments and research organisations etc. As with many sectoral 

developments, particularly in the marine environment, this information has not 

been widely disseminated, reviewed or synthesised for a wider audience or been 

made publicly available. 

Consequently, the Research Advisory Group on Offshore Renewable Installations 

identified a need for Guidance on seabed installation techniques and their 

potential environmental effects. This guidance is required principally to inform 

wind farm developers and others during the Environmental Impact Assessment 

(EIA) and determination stages of the consents process. The guidance will be 

equally of use to other burgeoning marine renewable energy technologies, such 

as wave and tidal. 

1.2.1 STUDY AIMS 

The study aims to provide an information resource and guidance to government 

and developers on the range of cable installation techniques available, their 

12

Introduction 

likely environmental effects and potential mitigation, drawing on wind farm and 

other marine industry practice and experience. 

Through the collation of existing information and experience from a range 

of sources, the guidance will assist government, developers, stakeholders 

and regulators during the EIA and consenting process including stages of 

information provision, review and approval of such information. Importantly, 

an understanding of the difficulties and constraints of cable installation has 

been provided such that impacts can be avoided, reduced or minimised. This 

document also deals with the practical application of the installation and 

mitigation techniques available to developers so that the most relevant and 

up-to-date technology can be applied in the most appropriate situation. 

In summary, the specific objectives for this project are as follows: 

● 

● 

● 

● 

To summarise the range of types of cables and small diameter pipelines 

currently installed in the EU shelf marine environment; 

To discuss the range of techniques used to install and maintain the 

aforementioned cables and pipelines; 

To summarise the environmental impacts that could arise as a result of 

these installation and maintenance techniques (both reported and potential 

impacts); and 

To produce a technical report document which reviews cabling techniques 

and their environmental effects for use in the development of wind farm 

projects (and other marine renewable technologies). 

1.3 Report Format 

Section 1 describes the project’s background, aims and objectives. 

Section 2 outlines the relevant legislation applicable to the development of 

offshore wind farms and the installation/maintenance of cables/pipelines. 

Section 3 summarises the types of cables and pipelines that are currently 

installed in the marine environment and the range of installation techniques 

that are available. The relevant characteristics of each type are provided, such 

as dimensions, construction material, armouring, depth of burial, additional 

protection measures, normal service life and decommissioning. The range of 

installation techniques that are available are also described as well as the type 

of engineering standards and constraints that might be of relevance (i.e. depth 

of burial in areas of sediment movement or where beam trawling occurs). 

Section 4 outlines the physical changes or effects to the seabed and surface 

sediments that could be expected to occur during cabling activity and the key 

characteristics that influence the extent of change. 

13



Section 5 describes the potential environmental impacts expected during 

cabling activities together with possible mitigation measures that could be used 

to reduce their scale of effect. Impacts are described and discussed for: 

● 

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14 

Intertidal habitats; 

Subtidal ecology; 

Natural fish resource; 

Commercial fisheries; 

Marine mammals; 

Ornithology; 

Shipping and navigation; 

Seascape and visual character; and 

Marine and coastal archaeology. 

Section 6 provides examples of good practice measures during cabling activities 

which could be adopted in conjunction with mitigation measures to minimise 

the magnitude and significance of effects on the local environment. 

Section 7 outlines the gaps in understanding and data identified as part of this 

project. 

Section 8 provides a full reference list.

2 Legislation 

2.1 Introduction 

This section provides an overview of the key UK legislation regulating 

developmental activities during the construction, operation and decommissioning 

phases of cabling activities associated with offshore wind farm developments. 

2.2 Licences and Consents 

A statutory consenting process exists through which offshore renewable 

development applications are considered in relation to UK policy aims and 

international obligations. The consenting process is designed to ensure that 

each development decision is made on the basis of a comprehensive balanced 

consideration of impacts, both positive and negative. 

The consents process in England and Wales is managed by the Department 

for Business, Enterprise and Regulatory Reform’s (BERR) Offshore Renewables 

Consents Unit (ORCU) and other statutory consenting authorities (the Department 

for the Environment, Food and Rural Affairs (Defra), the Department for Transport 

(DfT) and the National Assembly for Wales (NAW)). The Scottish Executive is 

responsible for administering the consents process for offshore wind farms in 

Scottish waters, and in Northern Ireland, the Department for Enterprise, Trade 

and Industry (DETI). The above organisations deal with consents received 

under the Electricity Act (EA), the Transport and Works Act (TWA), Food and 

Environment Protection Act (FEPA) and Coast Protection Act (CPA). 

The consents process for offshore wind farms, in England, Scotland and Wales, 

is explained in the DTI’s (now BERR) Strategic Planning Framework consultation 

paper (2002) and full guidance for the consents process is provided within DTI 

(now BERR) guidance notes (2004). National Planning Policy Guidelines (NPPG) 

6 sets out national planning guidance for renewable energy developments in 

Scotland (Scottish Natural Heritage, 2004). In Northern Ireland, the DETI has 

produced a Strategic Energy Framework for Northern Ireland (DETI, 2004) and 

the Department of the Environment within the Northern Ireland Assembly has 

published details on planning process in Planning Policy Statement 1 – ‘General 

Principles’ (Department of the Environment, 1998). 

Under the auspices of the Electricity Works (Environmental Impact Assessment) 

Regulations 2000 (SI 2000/1927), applications for consent for offshore wind 

farms (greater than 1MW) must be accompanied by an Environmental Statement 

(ES), the formal presentation of the Environmental Impact Assessment process. 

In Scotland, this falls under the Electricity Works (Environmental Assessment 

(Scotland) 2000 Regulations (SI 2000/320) and in Northern Ireland, the Electricity 

(Northern Ireland) Order 1992 (SI 1992/231). The ES details the nature and 

extent of the potential impacts of each aspect of the construction, operation 

and eventual decommissioning of the wind farm and associated infrastructure. 

15



It is from the evidence presented in the ES, and subsequent critical analysis 

by a range of appropriate authorities, that the necessary consents are granted 

and any conditions with regard to mitigation, monitoring and best practice are 

applied. 

Offshore cabling applications and offshore wind farm applications require 

developers to obtain a number of consents. A summary of these consents are 

set out in Table 2.1. The standards and codes of practice necessary for subsea 

cabling activities for the development of an offshore wind farm are provided in 

Appendix A. 

Specifically, for cabling activities associated with the offshore wind farm industry, 

a FEPA and CPA licence is required for: 

● 

● 

16 

Installation of cables; and 

Deposition of material for cable/scour protection purposes such as rock 

armouring and grout bags. 

Under FEPA, the decision as to whether a licence will be granted will depend 

on the assessment by the Licensing Authority of potential hydrological effects, 

risk to fish, mammals and other marine life from contaminants, noise and 

vibration, effects from potential increases in turbidity, smothering and burial of 

marine life, any adverse implications to designated marine conservation areas 

or interference to other marine and coastal users. 

Under CPA, the Secretary of State must determine whether marine works will 

be detrimental to the safety of navigation in relation to the cabling installation/ 

removal activities. 

Guidance for EIA in respect of FEPA and CPA requirements is provided by CEFAS 

(CEFAS, 2004). 

If an offshore wind farm development is planning a high voltage electrical 

connection to the onshore grid, then use of the transmission assets involved – 

the offshore substations, related electrical plant, and export cables – will require 

a transmission licence under the Electricity Act 1989. A distribution licence would 

be required if the electrical connection is at low voltage. These licensing regimes 

are currently under development and are likely to be introduced in 2008 and 2007 

respectively. Further information on obtaining licences for offshore transmission 

and/or distribution can be obtained from the Offshore Transmission Team at 

BERR and on the BERR website.

Legislation 

Table 2.1: Statutory licences and consents for offshore wind farms 

Act of Parliament Consent type Competent Authority 

Section 36 – Electricity Act (EA) 1989 

(as amended by the Energy Act 2004) 

Electricity (Northern Ireland) Order 

1992 

Section 37 – Electricity Act (EA) 1989 

Section 5 and 6 – Electricity Act (EA) 

1989 

Section 5 – Food & Environment 

Protection Act (FEPA) (Part II) 1985 

Section 34 – Coast Protection Act 

(CPA) 1949 

Section 90 or Section 57 – 

Town and Country Planning Act 1990 

Section 57 or Section 28 – Town and 

Country Planning (Scotland) Act 1997 

Section 109 – Water Resource Act 

1991 

Section 20 – Water Environment and 

Water Services (Scotland) Act 2003 

For offshore wind power generating 

stations within territorial waters adjacent 

to England and Wales. 

For overhead electric power lines 

associated with offshore wind farm 

projects. 

For the transmission or distribution of 

electricity. 

For depositing articles or material in 

the sea/tidal waters below mean high 

water springs (MHWS), including the 

placement of construction material or 

disposal of waste dredging. 

Construction under or over the seashore 

lying below MHWS. To make provision 

for the safety of navigation in relation to 

the export cable route and inter-turbine 

cabling network 

Deemed planning permission sought 

as part of the Section 36 applications 

for the onshore elements of the cable 

route. Section 57 – planning permission 

is sought separately, from the relevant 

local planning authority. 

To erect a structure e.g. cabling in, over 

or under a water course that is part of a 

main river 

Transport and Works Act 1992 Relating to offshore generating stations 

and works which may interfere with 

rights of navigation. 

Source: Adapted from DTI guidance (2004) 

BERR (England & Wales) 

Scottish Executive (Scotland) 

DETI (Northern Ireland) 

Ofgem 

Marine Consents Environment 

Unit (MCEU) of DEFRA 

Environment and 

Heritage Service (EHS) 

of the Department of the 

Environment (DOENI) 

Scottish Office of Agriculture, 

Environment and Fisheries 

Department (SOAEFD) 

MCEU 

EHS (DOENI) 

SOAEFD 



DETINI (Northern Ireland) 

Environment Agency 

(England and Wales) 

Scottish Environment 

Protection Agency (SEPA) 

(Scotland) 

Rivers Agency (Northern 

Ireland) 



DETINI (Northern Ireland) 

17

3 Cable types and 

18 

installation techniques 


This section of the report lists all of the cable types which are anticipated to be 

used in the near future for offshore wind farm development work and sets the 

background to the methodologies which have been developed in other industries 

for the safe installation and protection of subsea cables. It then outlines the 

various cable protection methods, including all of the various cable burial 

methodologies together with details of alternative external cable protection 

systems. This includes details of the cable protection methods which are most 

applicable and relevant for the offshore wind farm industry. Information on 

burial assessment survey techniques which are commonly used as a pre-survey 

activity before the main cable installation is included. Finally, this section reviews 

how legislation on decommissioning of the installed cable systems could also 

give rise to procedures which could have an impact on the environment. 

3.2 Cable Types 

3.2.1 GENERAL 

This section of the report contains details of offshore subsea cables which would 

typically be used for offshore wind farms. It also contains details of the cables 

which are commonly used in the subsea telecommunications and power cable 

industries. 

The design and construction of a particular subsea cable will be directly governed 

by the functionality of the cable and any requirement to protect the cable from 

any potential hostile seabed intervention. Figure 3.1 illustrates a typical range 

and size of cables which can be used for a variety of different applications. Some 

of the cables included in this figure are not all subsea cables. 

3.3 Offshore Wind Farm Cables 

3.3.1 GENERAL 

The design of subsea cable systems for offshore wind farms will be influenced 

by a number of factors. Some of the factors are generic, while others are projectspecific:

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Cable types and installation techniques 

Connection voltage – Offshore wind farm projects need to be connected to 

regional distribution networks, rather than to the national transmission system. 

In England and Wales, these distribution networks currently include 132kV 

and 33kV systems. Some of the early Round 1 offshore wind farm projects 

have made connections at 33kV while several others have connections at 

132kV. It is probable that all Round 2 sites will secure connections at 132kV. 

Cable design – Three-core subsea cables using solid insulation (ERP or XLPE) 

are typically used for operation at voltages up to 132kV. Higher voltage cables 

that use oil as an insulating medium are not deemed to be environmentally 

acceptable owing to the potential risks associated with oil leakage in the 

near shore environment. The cables are typically designed to give a power 

transmission capacity of up to 40MW for a single 33kV cable and 160MW for 

a single 132kV cable. 

Turbine size – Offshore wind farm developers are continually looking to the 

turbine manufacturers to develop larger and more energy efficient turbines. 

The early UK Round 1 developments such as North Hoyle, used 2.5MW wind 

turbine generators (WTGs). Current offshore wind farm developments are 

planning to use 3.0MW and 3.6MW machines and plans for a 5.0MW machine 

are already well advanced. 

Distance from shore – The use of a single 132kV cable to shore provides a 

cost-effective alternative to the use of three or four 33kV cables. The installed 

cost (per kilometre) of a single 132kV cable is considerably lower than the 

installed cost of three 33kV cables, but this solution requires an offshore 

substation in order to step up to 132kV from the wind farm collection voltage 

(usually 33kV). Some of the current projects under consideration have more 

than one 132kV export cable to link the offshore wind farm to shore. 

19



Figure 3.1: Diverse Range of Different Cable Types 

3.3.2 EXPORT CABLES 

The function of the export cables is to transmit the electrical power from the 

offshore wind farm to the appropriate cable connection facility at the shoreline 

or landfall. The electrical parameters of these cables will depend on the choice 

made between two options: 

Option 1 – No voltage step-up offshore. The power produced from the offshore 

wind farm is transmitted to shore at the collection system voltage. With this 

option there is no need for an offshore substation. A number of the offshore 

turbine structures act as a localised hub from which the export cables then 

transmit the generated power back to shore. 

Option 2 – Voltage step-up offshore. The power produced by the wind farm 

is transmitted to shore at a different voltage level, greater than the collection 

voltage. An offshore substation, containing one or more step-up transformers, 

is required. The offshore substation acts as the collection point within the wind 

farm. The export cables then run from the offshore substation to shore. 

20


High voltage direct current (HVDC) is not economically viable at present due to 

the high cost of HVDC converters, however it may, in the future, be used for sites 

situated further offshore. 

The current and future generation of export cables are likely to be rated with 

a voltage of 132kV. The cable is likely to be constructed with 3 core copper 

conductors, insulation and conductor screening, steel wire armour and either 

ERP or XLPE insulation with a lead sheath. The copper conductor size is typically 

estimated at between 300mm 2 and 1200mm 2 . The cables will contain optical 

fibres embedded between the cores for data transmission and communications. 

The range of indicative cable conductor sizes and overall dimensions which 

could be used for a 132kV export cable for an offshore wind farm are shown in 

Table 3.1. 

Table 3.1: Typical Cable Characteristics For 132kv Cable 

Details 132kV Cable Type 

300 mm² 500 mm² 800 mm² 1000 mm² 1200 mm² 

Overall Diameter (mm) 185 193 214 227 232 

Weight (kg/m) 58 68 88 100 108 

MVA (approx) 127 157 187 200 233 

3.3.3 INTER-TURBINE ARRAY CABLES 

The inter-turbine array cables are the cables which connect the offshore turbines 

into arrays and also connect the various arrays together. It is normal practice 

to cable several turbines together in an array, with each cable providing a link 

between two adjacent turbines. Each end of the cable is terminated onto the 

high voltage (HV) switchgear located within the turbine tower. Because they 

connect to the HV switchgear at the turbines, the operating voltage for the 

inter-turbine cables is limited to 36kV. These cables would also connect any 

offshore substation to the offshore WTG arrays. The cables between WTGs are 

relatively short in length (typically in the range 500m to 950m). However, the 

cables between the offshore substation and the WTG arrays could be longer and 

possibly up to 3.0km. 

The inter-turbine array cables will typically be 33kV, 3-core copper conductors 

with insulation/conductor screening and steel wire armoured. The insulation 

will be either dry type XLPE, wet type XLPE or a combination of both. All cables 

will contain optical fibres embedded between the cores. A number of conductor 

sizes would be used depending on the load current that the cable is required to 

carry. The ranges of indicative cable conductor sizes and overall diameters that 

may be used are shown in Table 3.2. 

21



Table 3.2: Typical Cable Characteristics for 33kV cables 

Details 33kV Cable Type 

22 

95 mm² 240 mm² 400 mm² 630 mm² 800 mm² 

Overall Diameter (mm) 89 104 127 143 153 

Weight (kg/m) 12.2 18.6 38 49 59 

MVA (approx) 18 29 36 44 48 

3.4 Telecoms Cables 

This section outlines the types of cables which are commonly used by the subsea 

telecommunications industry to install their networks on a worldwide basis. It is 

normal practice for the subsea telecommunications industry to conduct a Burial 

Assessment Survey in advance of the design of the cable type along the full route 

of the cable (see Section 3.10). The Burial Assessment Survey will be used to 

assess the level of armouring required for a section of the cable length together 

with the proposed depth of burial. Many of these cable systems can be in excess 

of thousands of kilometres in length, therefore, the design of armouring for the 

cable can have a significant influence on the overall cost of the cable. There is also 

a relationship between the design of outer protective armouring and the proposed 

depth of burial to protect the cable in its in-service condition. Hence, the results 

of the Burial Assessment Survey are used to identify what level of burial can be 

achieved along the cable route. This assessment will take into account the burial 

system to be employed on the project together with the design armouring for the 

cable. 

Section 3.10 of this report reviews Burial Assessment Surveys in more detail. 

Table 3.3 gives typical armoured cable characteristics for a number of subsea 

telecommunication cables. The characteristics of these cables are set out in 

detail in the following sub-sections. 

Table 3.3: Typical Characteristic for Subsea Telecommunication Cables 

Characteristic Unit Lightweight 

Armour 

(AL) 

Single 

Armour 

(A) 

Double 

Armour 

(AA) 

Lightweight cable diameter mm 21.5 21.5 21.5 

First lay steel wire diameter mm 4.0 4.9 4.9 

Number of steel armour wires in the lay 19 16 16 

Pitch of armour wire mm 530 539 539 

Second lay mild steel wire diameter mm - - 7.0 

Number of steel wires in the lay (left hand) - - 18 

Pitch mm - - 610 

Outside diameter mm 36.7 38.5 57.7 

Weight in air kg/m 3.0 3.5 9.7 

Weight in water kg/m 1.9 2.9 7.0

3.4.1 LIGHTWEIGHT CABLE 


Lightweight cable, as its name suggests, is cable which does not have any 

form of outer protective armouring. This type of cable is typically used in very 

deep water (usually in excess of 1000m) where the cable risk assessment has 

identified that the cable is highly unlikely to be subject to any form of hostile 

seabed intervention (e.g. anchoring or trawling). 

3.4.2 ‘SHARK BITE’ CABLE 

Shark Bite cable was initially developed by AT&T in the USA when they 

discovered that the electromagnetic field emanating from the power feed 

system in their subsea telecommunication cables was agitating the local shark 

population. The sharks would then attempt to bite any surface laid sections of 

cable. To protect the cable, a new design, which provided a light single armour 

around the cable adequate to repel any potential shark attack, was derived. The 

cable also had additional screening installed in the cross sectional make-up in an 

attempt to reduce the amount of electro magnetic field propagation. 

3.4.3 LIGHTWEIGHT ARMOURED CABLE 

Lightweight armoured cable is a lighter version of single armoured cable where 

the steel armour wires would typically be 4mm in diameter, as opposed to the 

4.9mm diameter steel wires which are used in the conventional armoured cable 

(Figure 3.2). A typical application of lightweight armour would be for a deep sea 

section of cable which was going to be laid over a rocky seabed area and where 

the risk of abrasion damage to a lightweight type cable was considered to be 

high. 

3.4.4 SINGLE ARMOURED CABLE 

Single armoured cable is a cable type which is typically used in conjunction with 

cable burial as a means of providing overall protection for the installed cable 

(Figure 3.3). This type of cable would be employed in areas where the risk from 

hostile seabed intervention such as fishing had been identified. This may occur 

in water depths anywhere between 50m to 1000m. 

3.4.5 DOUBLE ARMOURED CABLE 

Double armoured cable would typically consist of an inner armoured layer 

(similar to the single armoured cable type) with steel wires of 4.9mm in diameter, 

which would then be overlaid with a second layer of armouring with steel wires 

of 7mm in diameter (Figure 3.4). Double armoured cable is significantly heavier 

and more inflexible than the single armoured cable which makes it more 

difficult and expensive to produce and install. Double armoured cable would 

typically be employed where there is a perceived high risk that the cable may 

23



not achieve the target depth of burial, where there is risk of crush damage in 

areas which are known to be heavily trawled, or across busy shipping lanes or 

navigational channels (i.e. at any location where the perceived risk of hostile 

seabed intervention is high). 

Figure 3.2: Typical Cross Section of a Lightweight Armoured 

Cable (used in deepwater (>1000m) with low risk of hostile seabed 

intervention) 

24


Figure 3.3: Typical Cross Section of a Single Armoured Cable (used 

in water depths of 50-1000m, where hostile seabed intervention is an 

identified risk) 

25



Figure 3.4: Typical Cross Section of a Double Armoured Cable (used 

in areas subject to a high risk of hostile seabed intervention) 

3.4.6 ROCK ARMOURED CABLE 

Rock armoured cable is a double armoured cable where the outer layer of contrahelically 

laid armour has a tight pitch angle which results in a tight packing of 

the armour wire around the cable. This armour type provides a very stiff outer 

protection to the cable core and this type of cable would typically be employed 

where the cable would be laid over rocky outcrops in shallow water, where the 

cable was perceived to be at high risk from abrasion damage, or other localised 

risks resulting from a surface laid cable in a shallow water environment. 

3.5 Power Cables 

Power cables are defined as subsea cables which are used to either import or 

export power capacity. For example, there are a number of installed subsea 

power cables between the UK and France which allow for such power exports 

and imports. Similar systems are used to connect island settlements around 

the UK such as the Scottish Isles and the Isle of Wight. These cables are large 

diameter cables with similar characteristics and behaviour to the export cables 

associated with offshore wind farm developments. 

26


Power cable diameters can range from 75mm in diameter up to 240mm in 

diameter. The larger size being associated with 132kV export cables from 

offshore wind farm developments currently under consideration for Round 2 

developments. 

3.6 Flowlines, Umbilicals and Small Diameter Pipelines 

Flowlines, umbilicals and small diameter pipelines have been included in this 

review on the basis that some of the burial systems and other cable protection 

methods are commonly applied to this group of subsea products. 

These products fall into the larger diameter category (usually at least 200mm 

diameter) and are typically used to connect remote wellheads to offshore 

platforms and to provide interconnect facilities between offshore installations. 

As well as containing fibre optics and power cores, these products have tubing 

within the make-up of the product which can be used for chemical injection and 

hydraulic power. 

The flowlines and umbilicals, by the nature of their make-up, are constructed 

using a specialist manufacturing process, are heavy, have stiff properties and 

also have a large minimum bend radius. 

3.7 Background to Safe Installation and Protection of Subsea 

Cables 

This section sets out the history and background to the need for protection 

methods for subsea cables. 

3.7.1 THE NEED FOR CABLE PROTECTION 

Cable burial and other protection measures are used to ensure cables are 

adequately protected from all forms of hostile seabed intervention. If a cable is 

not adequately protected, damage can and will occur. Figure 3.5 illustrates the 

results of seabed deployed fishing gear becoming entangled with an unprotected 

cable system. Figure 3.6 shows a typical deployment of bottom trawl fishing 

gear and Figure 3.7 shows how a trawl or otter board can potentially engage and 

snag a subsea cable giving rise to the damage as illustrated in Figure 3.5. 

27



Figure 3.5: Fishing Gear Entangled with an Unprotected Cable System 

28

Figure 3.6: Typical Deployment of Bottom Trawl Gear 


Figure 3.7: Schematic to Illustrate Potential Damage to Cable as a 

Trawl / Otter Board Engages a Subsea Cable 

29



3.7.2 HISTORY 

The history of safe installation and protection of subsea cables dates back to 

the 19th Century when early telegraph cables across both the North Sea and 

the Atlantic were subject to frequent damage from the local fishing activity and 

other vessels dragging their anchors across the seabed. In this respect, little has 

changed in over 250 years and all seabed cables and pipelines must be assessed 

in terms of the risks of potential damage. 

It was only in the early 1980s that effective cable burial became a primary 

method for protecting subsea cables. The pioneers in this field were the owners 

of the various offshore submarine telecommunication networks who realised 

that the new fibre optic systems would require a robust protection system to 

prevent costly interruptions to service. The new cables were being designed 

to carry many thousand more circuits than their old analogue predecessors. 

British Telecom conducted extensive research programmes through their 

Martlesham Research Facility, where a number of onshore and offshore trials 

were undertaken to establish the effectiveness of cable burial with varying 

depths of burial and cover. The tests involved the use of different types of fishing 

gear, commonly used both in shallow and deep water applications, to simulate 

the real conditions of fishing gear passing over cables from different angles of 

approach. At the same time, Shell UK Exploration and Production (Shell) were 

conducting parallel tests to establish the potential effects of bottom trawl gear 

on medium to small diameter pipelines in the northern North Sea. The Shell 

study programme was subsequently supported by seven other North Sea oil and 

gas operators as a joint venture. 

3.7.3 SHELL STUDY PROGRAMME ON PIPELINES 

The results of the pipeline research showed that the concrete coated pipelines 

with diameters of 16 inches and above could be designed to tolerate trawl gear 

loads without the need to lower or cover the pipelines. However for smaller 

pipelines, or in the case where there was pipeline spanning, it was recognised 

that pipeline protection requirements for individual pipelines needed to be 

determined on a case by case basis. Subsequent to this research, a number of 

specialist pipeline ploughs and large track burial vehicles were developed, built 

and commissioned and put to effective use in the North Sea burying pipelines 

and flowlines for the offshore oil and gas industry. 

3.7.4 BRITISH TELECOM RESEARCH 

The British Telecom research, having focused on the much smaller targets of 

subsea cables, came to very positive conclusions that cables lowered into open 

trenches stood a much improved likelihood of being protected from seabed 

trawler gear. However, the best form of protection was afforded when a degree 

of cover was placed back over the trenched cable. In this latter condition the otter 

boards and trawl gear can easily pass over the cable without running the risk of 

30


snagging the cable and hence causing a fault. The early research showed that 

depths of trenching up to 300mm, with cover over the cable, would give a high 

level of protection to the subsea cable. However, the confidence in protection 

increased significantly with depth of burial and hence British Telecom initially 

specified a 600mm depth of burial for the early Trans-Atlantic, Cross Channel 

and North Sea cables. Figure 3.8 shows some of the research undertaken by 

British Telecom with a beam trawl passing over a telecom cable lowered in an 

open trench which is approximately 750mm wide and the cable is in the trench 

to a depth of approximately 250mm. 

Figure 3.8: Early Depth of Burial Research 

3.7.5 DEVELOPMENT OF SUBSEA CABLE PLOUGHS 

To achieve the target depth of burial of 600mm, as identified by the early British 

Telecom research, a number of narrow blade subsea ploughs were developed. 

These ploughs were designed to lift a wedge of soil, place the cable in the 

bottom of the cut trench and allow the displaced wedge of soil to naturally 

backfill over the cable. This concept is similar to the method used by British Rail 

to bury cables adjacent to railway tracks using a narrow blade plough deployed 

from a rail bogey. 

The new generation of subsea telecom cable ploughs were extensively land 

tested and then also trialled offshore before being put into active service. They 

became an instant success. The ploughs are towed by the cable laying vessel 

which simultaneously pays out cable as the ship makes forward progress 

31



thus ensuring the cable is simultaneously buried as the cable ship and cable 

plough make forward progress. The cable plough is fully instrumented and has 

hydraulically activated functions to allow for varying depth of burial. In addition, 

the instrumentation package ensures that all critical aspects of the operation can 

be controlled and monitored back on the host vessel as a real time operation. 

Between the late 1980s and the early 2000s, technologies improved the types 

and range of plough systems and these various types are described in detail 

in subsequent sections. During this same period of time, a number of tracked, 

remotely operated vehicles (ROVs) and free swimming ROVs were also 

introduced into the market. These vehicles were primarily used on projects 

where shorter lengths of cable burial were specified or where work in shallow 

water was required. They were also used where the ground conditions were 

beyond the capabilities of a conventional subsea plough and specialist cutting 

tools were required to cut into rock strata. 

3.7.6 CABLE FAULT ANALYSIS 

A number of studies have reviewed cable fault rates based on the depth of 

burial of the installed system. This research has shown that, on average, most 

‘hits’ on cables result from fishing activity and that surface laid cables would be 

regularly hit with fault occurrence directly related to the level of fishing activity. 

Cables buried to 0.6m depth are likely to only experience one or two hits in a 

10 to 15 year lifetime (probably in areas of shallow burial or where sand-waves 

are mobile) and cables buried to 1.0m are likely to have a high probability of 

remaining fault free. These statistics (which have been extracted from various 

papers submitted at SubOptic 1997 and SubOptic 2001) only provide guideline 

information and some systems are likely to remain fault free for their operational 

life. 

Various research has now resulted in some guidelines for the depth of burial 

when using ploughs, depending on the threat which the installed cable system 

may encounter during its operational life. Table 3.4 presents the target burial 

depth for subsea ploughs to place cables below the ‘threat line’ for the different 

hazards which are expected to pass over the installed cable and for varying 

seabed conditions. The figures quoted in the table include a 33% safety factor 

which recognises that the target depth of burial is not always achieved in the 

field for operational reasons. 

Figure 3.9 shows an average fibre optic fault rate (normalised per 1000km length), 

plotted against the age of the cables, for global cable systems installed in the 

period 1990 to 1999. Trends are plotted for cables installed in water depths up 

to 1000m and also for cables installed in water depths greater than 1000m. The 

decreasing trend in cable faults with the age of the cable is probably attributable 

to any exposed sections of cable being buried during the life of the cable. 

32


Table 3.4: Recommended Target Cable Burial Depths for Subsea 

Ploughs for Varying Seabed Conditions and Threats 

Threat Hard Ground 

(clay > 72kPa, rock) 

Trawl boards, beam trawls, 

scallop dredges 

Soft – firm soils 

(sand, gravel, 

clay 18-72kPa) 

Very soft – soft soils 

(mud, silt, 

clay 2-18kPa) 

< 0.4m 0.5m > 0.5m 

Hydraulic dredges < 0.4m 0.6m N/A 

Slow net fishing anchors N/A 2.0m > 2.0m 

Ships’ anchors 

Up to 10,000t DWT 

(50% of world fleet) 

Ships’ anchors 

Up to 100,000t DWT 

(95% of world fleet) 

< 1.5m 2.1m 7.3m 

< 2.2m 2.9m 9.2m 

Figure 3.9: Plot of Average Fibre Optic Fault Rate against the age of 

Installed Cable 

Source: Taken from Sub Optic 2001 Paper ‘Recent Trends in Submarine Cable Faults, Featherstone, Cronin, Kordahi 

and Shapiro’ 

3.7.7 SPECIALIST BURIAL TECHNIQUES 

In certain locations, highly specialist cable burial techniques have been developed 

to suit the exacting requirements of that particular location. For example, in 

places such as Hong Kong and Singapore Harbours where a number of subsea 

cables enter these communication hubs, it is vital that cables are protected from 

the potentially damaging effects of numerous anchor deployments. In these 

locations cable burial to depths of 5m and beyond have been specified, and 

achieved, using highly specialised burial equipment. 

33



3.7.8 CURRENT BURIAL TARGETS FOR THE OFFSHORE WIND FARM 

INDUSTRY 

The offshore wind farm industry has generally recognised that the main risks 

posed to the subsea cables derives largely from inshore fishing activity and 

dragging anchors from coastal vessel traffic. Typical target depths of burial 

between 1m and 2m have been specified for the majority of the Round 1 sites 

which have been developed to date. However, as the Round 2 developments 

move into deeper water, cable risk assessment may identify the need for 

increased burial depths as the possibility of anchor damage from larger seagoing 

vessels has to be considered. 

The use of burial assessment surveys may increasingly be used to establish 

target burial depths for cables. This procedure is explained in more detail in 

Section 3.10 of this report. 

3.7.9 REMEDIAL MEASURES 

Cable burial is the primary method for protecting subsea cables. Providing 

the correct burial machine is selected for the designated burial task, the target 

depth of burial is likely to be achieved. However, occasionally reduced burial or 

zero burial occurs. This will usually result as a consequence of an unforeseen 

event such as extreme weather; the departure from the scheduled installation 

procedure; extreme ground conditions which were not anticipated from the 

original survey; or equipment failure. Remedial measures will be required if the 

perceived risk for the installed cable is considered too high. In many cases it will 

not be possible to re-engage the cable with the original cable burial machine 

(i.e. subsea cable plough or tracked cable burial machine with enclosed cable 

path). In such cases the only remedial burial option available will be post lay 

burial by other means. Most post lay burial machines only utilise jetting systems 

which straddle the surface laid section of cable and these machines have 

limited capability when working in hard seabeds. Alternative cable protection 

methodologies which are reviewed in Section 3.8.6 can also be employed as a 

means of remedial protection for the installed subsea cable. 

3.8 Cable Protection Methods 

This section describes the methods which are currently used to protect subsea 

cables in the subsea telecommunications, power cable and oil and gas industries 

worldwide. 

Section 3.9 of this report focuses specifically on the cable protection methods 

which are relevant to the offshore wind farm industry. Section 3.9 does not repeat 

the methodologies as chronologically listed in this Section 3.8, but summarises 

the methodologies used on wind farms installed to date and also lists the cable 

protection methodologies considered relevant for current and future offshore 

wind farm developments. 

34

This section is divided into 2 sub-sections as follows: 

● 

● 


A review of all cable burial methods employed by all industries worldwide to 

protect subsea cables Sections 3.8.1 to 3.8.5 inclusive; and 

A review of alternative cable protection methodologies such as rock dumping, 

concrete mattresses, fronds, grout bags etc. Section 3.8.6. 

Each cable protection method is described in detail together with explanations 

of which methods are commonly employed for varying seabed condition. 

Illustrations and figures for the methods and equipment are provided to illustrate 

the methodology under review. 

3.8.1 CABLE BURIAL METHODS 

There are a diverse range of cable burial machines available in the market 

capable of burying and protecting offshore cables. This section of the report 

presents and describes these cable burial machines whilst also indicating the 

range of application and capabilities of each machine. This is always a subjective 

and sometimes controversial topic, as the performance of a cable burial machine 

in the offshore environment can vary from the supplier/owner specification 

which can tend towards more optimistic performance criteria. Therefore, 

the performance statements in this section of the report are based more on 

performance observed in the field by the authors and independent associates 

consulted, as opposed to the published performance criteria. 

In order to allow a comprehensive review of the various cable burial machines, 

and to be able to investigate the environmental effects associated with each 

machine, four categories have been established for the cable burial machines: 

● 

● 

● 

● 

Cable Burial Ploughs; 

Tracked Cable Burial Machines; 

Free Swimming ROVs with Cable Burial Capability; and 

Burial Sleds. 

All of the cable burial methods and cable protection methodologies which are 

described are used on a worldwide basis and on all different types of subsea 

cable systems. 

Table 3.5 sets out the various burial method options associated with each cable 

burial methodology and also provides a summary of the mode of operation. 

35



Table 3.5: Overview of Cable Burial Machines 

36 

Burial Machine 

Type 

Cable Burial 

Ploughs 

Tracked Cable 

Burial Machines 

Free Swimming 

ROVs with Cable 

Burial Capability 

OVERVIEW OF CABLE BURIAL MACHINES 

Burial Machine Options Mode of Operation 

Cable ploughs are available in 

varying types: 

● Conventional narrow share 

cable ploughs 

● Advanced cable ploughs 

● Modular cable ploughs 

● Rock ripping ploughs 

● Vibrating share ploughs 

Tracked cable burial machines can 

be equipped with the following 

burial tools: 

● Jetting systems 

● Rock wheel cutters 

● Chain excavators 

● Dredging systems 

Free swimming ROVs can be 

equipped with the following burial 

tools: 



Burial Sleds Burial sleds can be equipped with 

the following burial tools: 





Cable ploughs are towed from the host vessel with 

sufficient bollard pull to ensure continuous progress 

through the seabed with the cable being simultaneously 

buried as part of the lay process. The plough lifts a 

wedge of soil and places the cable at the base of the 

trench before the wedge of soil then naturally (via 

gravity) backfills over the cable. Cable ploughs are 

capable of working in a wide range of soils and are 

typically deployed where longer lengths of cable burial 

are required. Ploughs can operate in shallow water and 

in water depths up to 1500m. 

Tracked cable burial vehicles are usually operated and 

controlled from a host vessel such as a Dive Support 

Vessel (DSV) or a barge, have subsea power packs, and 

are controlled via an umbilical cable back to the host 

vessel. They usually operate in post lay burial mode 

(although one machine, the LT1, simultaneously lays 

cable from a reel mounted in the vehicle) and they 

are equipped with either jetting tools or mechanical 

cutting tools depending on the seabed conditions which 

are anticipated. The tracked cable burial vehicles are 

typically used on shorter lengths of cable burial work. 

Divers are often required to assist in the loading and 

unloading of cable into and out of the vehicle in the 

shallow water machines. However, some vehicles have 

fully automated cable loading/in-loading equipment. 

Some vehicles track over cables and straddle the cable 

with jetting forks. Tracked cable burial machines can 

be operated in shallow waters (providing motors and 

power packs can be cooled) and in water depths to 

2000m. 

Free swimming ROVs are operated and controlled 

from a host vessel such as a DSV or a barge. They will 

always operate in post lay burial mode and use either 

jetting or dredging system to bury subsea cables. 

Their range of application is limited to sands and clays 

(performance in clay will be directly related to available 

jetting power). ROVs are typically used on shorter 

lengths of subsea cable and can operate in water 

depths of 10m to 2500m. 

Burial sleds are usually operated in shallow waters for 

work in ports, estuaries, river crossings and shore-ends 

for cable systems. They are often deployed from barges 

or jack-ups and either have subsea power or utilise 

power systems which are mounted on the host vessel. 

The range of burial tools allows the varying types of 

burial sled to work in most seabed conditions from 

sands, gravels, clays and softer rock.

3.8.2 CABLE BURIAL PLOUGHS 


This section of the report reviews the various types of cable burial plough 

which are available for the burial of subsea cables. The mode of operation of 

each plough is described together with example types of each plough including 

details of manufacturers and owners. 

The different types of cable burial ploughs which are reviewed are as follows: 

● 

● 

● 

● 

Conventional Narrow Share Cable Ploughs; 

Advanced Cable Ploughs (including Modular Cable Ploughs); 

Rock Ripping Ploughs; and 

Vibrating Share Ploughs. 

The general principle of operation of a cable plough is illustrated in Figure 

3.10. 

Figure 3.10: Illustration of Cable Burial through a Subsea Cable 

Plough 

It is estimated there are now approximately 30 conventional narrow share and 

approximately 25 advanced cable ploughs (advanced, rock ripping and vibrating 

share) currently in service around the world. 

Conventional narrow share cable ploughs 

This type of cable burial plough is a passive tool towed from a host vessel. The 

plough share cuts a wedge of soil which is then lifted by the action of the plough 

37



cutting through the seabed. The cable is then fed through the pathway in the 

plough and the wedge of soil placed as cover over the cable. The cable laying 

operation involves cable being laid from the host vessel (usually over the stern 

of the vessel) with a managed catenary down to the bellmouth entry in the cable 

plough. The simultaneous lay and burial of the cable is then achieved as the 

cable vessel makes forward progress and the cable plough buries the cable as 

described above. Depth of burial is controlled by varying the deployed position 

of the hydraulically controlled skids and the plough is fully instrumented with 

cameras and sensors to monitor the passage of the cable through the plough 

and to also measure pitch and roll, depth of burial, distance travelled by the 

plough and residual tension in the installed cable. Figure 3.11 shows a typical 

narrow share plough with the range of sensors and components illustrated. 

Figure 3.11: Typical Narrow Share Cable Plough Illustrating Sensors 

and Plough Components 

These types of plough can typically achieve a depth of burial of up to 1.0m in a 

wide range of seabed conditions and have been successfully used on telecom, 

power cable and UK offshore wind farm projects. 

A selection of commonly used narrow share cable ploughs including details of 

the manufacturers and owners is listed in Table 3.6. 

38


Table 3.6: Selection of Commonly Used Narrow Share Cable Ploughs 

PLOUGH CONVENTIONAL NARROW SHARE CABLE PLOUGHS 

Type Maximum Burial Depth Operator Manufacturer 

Cable ploughs (several ploughs 

deployed worldwide) 

1.0m to 1.5m Global Marine Systems SMD Hydrovision Ltd 

Sea Stallion Cable Plough 2.0m Bohlen & Doyen The Engineering 

Business Ltd 

Sea Stallion Cable Plough 2.0m Originally Cns Ltd The Engineering 

Business Ltd 

Sea Plows (several ploughs 

deployed worldwide) 

1.5m Tyco Submarine 

Systems 

SMD Hydrovision Ltd 

Figure 3.12 shows a cable plough being deployed from a purpose built launch 

and recovery system installed on the deck of the host vessel. Figure 3.13 shows 

a conventional narrow share cable plough burying an offshore wind farm power 

cable away from the shoreline at North Hoyle. This figure clearly illustrates that 

only a minimum disturbance occurs on the beach or seabed during a cable 

burial operation using this type of plough. The plough share is deployed to a 

maximum depth of penetration of approximately 2.0m. The ‘disturbed’ mounds 

of sand adjacent to the cut trench are approximately 300mm to 400mm high. 

Figure 3.14 shows the same cable plough burying the North Hoyle Offshore Wind 

Farm export cable up the beach with the offshore wind farm in the distance. 

Figure 3.12: Cable Plough Launch and Recovery from a Host Vessel 

(Photo courtesy of The Engineering Business Ltd) 

39



Figure 3.13: Conventional Narrow Share Cable Plough Burying 

a North Hoyle Offshore Wind Farm Export Cable 

Figure 3.14: Sea Stallion 4 Cable Plough Burying North Hoyle 

Export Cables up the Beach 


40

Advanced cable ploughs 


The category of Advanced Cable Ploughs covers the new generation of cable 

ploughs which have been designed to achieve increased burial depth for subsea 

cables of up to depths of 3.0m. Table 3.7 below lists a number of Advanced 

Cable Plough types. 

One type of Advanced Cable Plough system utilises a number of water jets 

fitted within the plough share to fluidise material at the leading edge of the 

share which in return reduces the required tow force and allows the plough 

share to penetrate deeper into the seabed. These new multi-depth ploughs have 

the ability to bury cables up to 3.0m deep. The water jets are most effective in 

sands, gravels and weaker clay conditions and have limited use in harder seabed 

conditions. 

An alternative Advanced Cable Plough type is the Sea Stallion cable plough. This 

large cable plough can bury cables up to 3.0m deep by using a plough share 

with a unique geometry to allow this deeper cable burial. Figure 3.15 shows 

a Sea Stallion cable plough on the quayside with the double cutting plough 

shoe design and Figure 3.16 shows the same plough about to commence burial 

operations from the beach. 

Table 3.7: Advanced Cable Plough Systems. 

PLOUGH ADVANCED CABLE PLOUGHS 


MD3 – jet assisted plough 3.0m CTC Marine Projects SMD Hydrovision Ltd 

Advanced cable plough – deep 

burial plough 

Hi – Ploughs – standard share or 

injector share 

Up to 3.7m CTC Marine Projects SMD Hydrovision Ltd 

2.0m standard share 

3.25m injector share 

Global Marine SMD Hydrovision Ltd 

Advanced Sea Stallion Up to 3.0m Various The Engineering 

Business Ltd 

Sea Plan 8 Up to 1.5m Tyco Submarine 

Systems Ltd 

Perry Slingsby 

41



Figure 3.15: Sea Stallion Cable Plough on Quayside 


Figure 3.16: Sea Stallion Cable Plough 

Commencing Burial from the Beach 


42


Another type of Advanced Cable Plough is the modular cable plough, the concept 

for which was originally conceived in the 1980s. The principle of the modular 

plough is to have a plough system with an interchangeable plough share which 

allows the operator to either fit a narrow share for cable burial or to fit a modular 

V-type share for the burial of flowlines or small flexible pipelines. 

This type of plough has not proved popular and most operators have opted to 

design and build a plough system specifically tailored to the market they wish 

their plough system to operate in. An example of a modular plough is detailed 

in Table 3.8 below. It is believed that this plough system has only been used in 

cable burial mode; however, this has not been confirmed. 

Table 3.8: Modular Cable Ploughs 

PLOUGH MODULAR CABLE PLOUGHS 


Modular plough system 

(cables or pipes) 

Rock ripping ploughs 

1.2m CTC Marine Projects SMD Hydrovision 

Ltd 

The Rock Ripping Ploughs were developed in response to requirements from 

the subsea telecommunications market to provide a capability to protect 

cables in areas where telecom cables passed over seabeds which consisted of 

outcropping rock, or where the seabed strata was exceptionally hard and outwith 

the capabilities of a conventional narrow share plough. This requirement was 

primarily driven by fishermen trawling in seabed areas where such seabed 

conditions exist. Historically the fishermen would have avoided such areas 

owing to the risk of snagging gear. However, the need to find new fishing 

grounds has now forced the fishermen to work in these more difficult areas and 

in deeper waters. 

Historically when a cable had to be buried in rock the traditional approach would 

have been to use a rock wheel cutter. These tools can cut an efficient trench, 

but tend to make relatively slow progress depending on the nature of the rock. 

Furthermore, the wear rate on the cutting teeth can be substantial. Therefore, the 

rock ripping plough was developed to meet this new challenge. 

A Rock Ripping Plough has an additional, folding, rock penetrating tooth which 

is fitted to the leading edge of the plough share. The rock penetrating tooth 

significantly increases the ability of the rock ripping plough to penetrate and cut 

trenches in very hard ground seabed conditions. The overall performance of the 

plough is not generally affected in any way when the plough is cutting the trench 

in rock ripping mode. 

Table 3.9 lists a number of rock ripping ploughs which are currently available in 

the marketplace. Figure 3.17 shows a rock ripping plough; the rock penetrating 

tooth is clearly visible on the leading edge of the plough. 

43



Table 3.9: Rock Ripping Ploughs 

44 

PLOUGH ROCK RIPPING PLOUGHS 


Rock Plough 1 1.0m rock 

3.0m soft soils 

Rock Plough 2 1.0m rock 

3.0m soft soils 

ISU Plough 

(for offshore umbilicals) 

Figure 3.17: Rock Ripping Plough 

(Photo Courtesy of CTC Marine Projects) 

Vibrating share ploughs 

CTC Marine Projects SMD Hydrovision Ltd 

CTC Marine Projects SMD Hydrovision Ltd 

1.0 to 1.2m CTC Marine Projects SMD Hydrovision Ltd 

A vibrating share plough consists of a narrow share (suitable for the burial 

of cable systems) which is then vibrated to ensure cutting progress through 

difficult seabed conditions such as chalk and gravel beds. The vibration action is 

usually achieved by a hydraulic actuator. Table 3.10 lists examples of vibrating 

share ploughs.

Table 3.10: Vibrating Share Ploughs 


PLOUGH VIBRATING SHARE PLOUGHS 


ESV 12 Up to 2.2m Travocean Unknown 

ESV 11/3 

Shallow Water Only 

(Up to 3 cables simultaneously) 

ESV 07/3 

Shallow Water Only 

(Up to 3 cables simultaneously) 

Up to 1.3m Travocean Unknown 

Up to 1.1m Travocean Unknown 

The Travocean ESV 12 vibrating plough was successfully deployed to bury the 

Tangerine telecom cable off the east Kent coast adjacent to Dumpton Gap. The 

subsea cable system was successfully buried to a consistent burial depth of 1.4m 

in a chalk seabed as confirmed by the cable owner. 

3.8.3 TRACKED CABLE BURIAL MACHINES 

There are a large number of tracked cable burial vehicles deployed around the 

world. These types of vehicles are generally divided into two sub-groups: 

● 

● 

Tracked Vehicles for Shallow Water Use (usually within the range of air 

divers); and 

Tracked Vehicles for Deep Water Use (in water depths up to 2500m). 

Table 3.11 describes a range of tracked cable burial vehicles used worldwide. 

45



Table 3.11: Tracked Cable Burial Machines 

46 

Type Burial Systems Available Maximum 

Burial 

Capability 

Eureka Jetting system 

Mechanical wheel cutter 

Mechanical chain excavator 

Otter Chain excavator for shallow water 

working 

Spencer Chain excavator for shallow water 

working 

1.0m 

1.2m 

2.2m 

Maximum depth of 

Operation 

2.2m Within diver 

operations 

1.5m Within diver 

operations 

Operator 

1500m Global Marine 

Global Marine 

Global Marine 

Nereus III Two separate burial tools available 1.0m 2500m Tyco Submarine 

Systems Ltd 

Nereus IV Two separate burial tools available 1.0m 2500m Tyco Submarine 

Systems Ltd 

CT2 Jet trenching system Up to 2.0m 

in sands 

and low to 

medium 

strength clays 

Trencher T2 Jetting system 


Trencher T1 Jetting system 


TM 03 Jetting system 



TM 02 Jetting system 



MED 1 Jetting system 


1.6m 

1.6m 

2.0m 

1.2m 

1.0m 

1.3m 

2.3m 

1.0m 

1.2m 

2.0m 

1.0m 

3.0m 

3000m CTC Marine 

Projects Ltd 

Up to 1000m CTC Marine 

Projects Ltd 

Up to 1000m CTC Marine 

Projects Ltd 

Within diver 

operations 

Within diver 

operations 

Travocean 

Travocean 

120m Travocean 

MED 1 Mechanical wheel cutter 1.5m 500m Travocean 

LBT1 Jetting system 



3.0m 

1.2m 

1.6m 

50m Marine Projects 

Int. 

Tracked cable burial vehicles are usually operated (but not exclusively) in post 

laid burial mode to bury subsea cables which have been previously laid on the 

seabed and are typically used for shorter sections of cable burial. However, 

some vehicles have been used successfully to bury up to 10km lengths of subsea 

cable. Tracked cable burial vehicles are launched from the host vessel and then 

locate the subsea cable system to be buried by using a combination of cable 

detection and underwater camera systems. The tracks then straddle the subsea 

cable and the cable is loaded into the cable burial tool which is fitted to the 

vehicle. As the vehicle makes forward progression the vehicle has the capability 

to automatically steer along the line of the cable with an auto track capability 

linked to the cable locator system fitted to the front of the vehicle.


Divers are often required to assist in the loading and unloading of cable into and 

out of the cable burial tool fitted to the tracked vehicles. However, some of the 

vehicles have automatic cable loading and unloading features with an auto eject 

capability in the event of a power system failure to the tracked vehicle during 

operation. Cable owners are often reluctant to use such systems and therefore 

the more complex burial tools tend to be limited to shallow water use where 

diver intervention is possible. In deeper waters (which are out of diver range) 

only the more simple burial tools such as a jetting tool consisting of two forks 

which are positioned either side of the cable, tend to be used. 

There are three types of burial tools which are commonly fitted to tracked cable 

burial vehicles: 

● 

● 

● 

Jetting systems (sometimes accompanied by a dedicated dredging system); 

Mechanical rock wheel cutters; and 

Mechanical chain excavator. 

These three types of burial tool are described below. 

Jetting systems 

Numerous types of jetting tools have been developed, most of which are 

subject to various patent protections, with operators usually favouring their own 

particular system. 

A jetting system works by fluidising the seabed using a combination of highflow, 

low pressure and low flow high pressure water jets to cut into sands, 

gravels and low to medium strength clays. Progress in clays is dictated by 

the available power budget to the tracked cable burial vehicle and the level of 

cohesion in the clay. In some cases a dredging system is employed to suck out 

the fluidised material to leave an open trench into which the cable then falls to 

the bottom through its self-weight. The jetting tools can also be fitted with a 

downwards forcing depressor which, together with the self-weight of the cable, 

helps to force the cable downwards in the fluidised trench. The effectiveness of 

any depressor system will be limited by the minimum bend radius and stiffness 

of the cable being buried and also by the on-bottom weight of the tracked cable 

vehicle itself to provide a downwards force onto the cable. This type of burial 

operation does give rise to sediments being suspended in the water adjacent 

to the burial operation and it can take a number of hours for such sediments to 

settle and for full visibility to return in the water column. 

Mechanical rock wheel cutters 

Mechanical rock wheel cutters can also be fitted to tracked cable burial vehicles 

and, as the name suggests, are used to cut narrow trenches into hard or rocky 

seabeds typically operating in the 1.5m trench depth range. The rock wheel 

47



cutter consists of a rotating wheel disc and is fitted with a number of replaceable 

rock cutting teeth. Progress is slow (100m/hr to 500m/hr) using these vehicles 

and, in extreme cases, progress of only tens of metres can be made in a full day 

of trenching operations. It is often the case that the vehicle has to be recovered 

for the cutting teeth to be replaced as they abrade quickly in the rock material 

thus losing their effectiveness in cutting mode. Owing to the very aggressive 

nature of the cutting tool, the cable is carefully guided through the tracked cable 

burial vehicle through an enclosed cable pathway over the top of the rock wheel 

cutter before being guided to the base of the cut trench. Diver intervention is 

usually required in any cable loading and unloading process. Figure 3.18 shows 

a shallow water tracked cable burial machine which is fitted with a rock wheel 

cutter, but has the flexibility to interchange to a mechanical chain excavator if 

required. 

Figure 3.18: Shallow Water Tracked Cable Burial Machine Fitted with 

a Rock Wheel Cutter 


Mechanical chain excavators 

Mechanical chain excavators are typically used in circumstances where the 

seabed material is beyond the capability of a jetting system or where deeper 

burial is required. Mechanical chain excavators typically operate in the 2.5m 

to 3.0m trench depth range as opposed to the 1.0m to 1.5m depth range that 

48


the mechanical rock wheel cutters operate in. The mechanical chain excavator 

tool usually consists of a number of cutting teeth similar to those used on the 

mechanical rock wheel cutter and a further number of mechanical scoops which 

are used to transport the cut material away from the trench. 

As with the mechanical wheel cutter, all the teeth on the mechanical chain 

excavator are replaceable and interchangeable depending on the seabed 

conditions. An auger is sometimes in place at the upper level of the mechanical 

chain excavator. The auger helps to move away the cut trench material and 

prevent it falling back into the trench or from clogging the cutting chain. The 

augers are usually short, and only move the cut seabed material approximately 

500mm away from each side of the trench. Again, because of the very aggressive 

nature of the cutting tool, the cable will be guided in an enclosed pathway around 

the top of the cutting tool to safeguard against any potential damage before it is 

placed into the base of the cut trench. Therefore, as for the wheel cutter, cable 

loading and unloading is a diver assisted operation. 

Simultaneous lay and burial vehicles 

In 2003, a new type of tracked cable burial vehicle was introduced to the offshore 

wind farm cable market, with the capability to lay and bury cables simultaneously. 

Manufactured by SMD Hydrodivision Ltd, the LBT1 was supplied to Marine 

Projects International Ltd and was first used on the North Hoyle Offshore Wind 

Farm for the burial of the inter-turbine array cables. Figure 3.19 below shows 

the LBT1 being deployed over the side of the host vessel with a full reel of cable 

to install a section of cable between two of the North Hoyle offshore turbine 

generators. The LBT1 was fitted with both a forward chain excavator system for 

working with hard soils and a jetting tool mounted to the rear of the vehicle for 

the burial of the subsea cable. 

49



Figure 3.19: LBT1 being deployed at the North Hoyle Offshore Wind 

Farm Site 

Previous Table 3.11 lists a selection of Tracked Cable Burial Machines, noting the 

various burial systems available for each machine together with corresponding 

details of burial capability. 

3.8.4 FREE SWIMMING CABLE BURIAL MACHINES 

A Free Swimming Burial ROV uses thrusters for propulsion and manoeuvrability 

and is equipped with a work package or work skid for intervention tasks or cable 

burial operations. All of the ROVs in use today are unmanned; however some 

of the early free swimming vehicles were manned. These vehicles are now 

consigned to museum exhibits. 

The early free swimming burial machines were manned autonomous vehicles 

with very limited power budgets for burial operations. The early Pieces vehicles, 

as they were called, were generally deployed on repair and maintenance duties 

and would spend extended periods trying to shallow bury exposed sections of 

subsea telecoms cable into the seabed. However, after a near fatal incident on 

an early CANTAT cable which ultimately resulted in a successful full scale rescue 

of a stranded Pieces vehicle, the industry looked to remotely controlled vehicles 

for the next generation of cable intervention. 

The first generation Free Swimming Burial ROVs were dedicated for repair and 

maintenance activities to assist in the repair of damaged sections of cable and 

50


to then attempt re-burial of the final repair splice of cable. The Free Swimming 

Burial ROVs used jetting systems to attempt the cable burial, but with low 

power budgets they only had limited success in sandy seabeds. SCARAB 1 and 

SCARAB 2 were the first vehicles of this type. Both of the vehicles were equipped 

with subsea power packs, control and telemetry functions via a control umbilical 

and had depth capabilities of 1000m. As subsea ploughs were introduced for the 

burial of subsea telecoms cables, the Free Swimming Burial ROVs developed into 

more sophisticated ROVs capable of a range of subsea intervention tasks with 

larger power budgets for cable burial. This next generation of Free Swimming 

Burial ROVs had cable burial tools with low and high pressure jetting tools and 

dredge units to allow the vehicles to work in sandy, clay and gravel seabeds. 

The SCARAB 4 vehicle which came into service in 1990, was fitted with a state 

of art rotating cylinder jetting tool with a series of high pressure jets capable of 

working into 400KPa clays. 

Free Swimming Burial ROVs have generally stayed in the niche market area of 

cable repair and maintenance. However, these vehicles have seen significant 

technological development with jetting tools now capable of deeper burial using 

“jetting swords” which sit either side of the cable to be buried, and with depth 

ratings down to 2500 and 3000m. 

Table 3.12 provides a listing of a number of free swimming cable burial vehicles 

which are used worldwide. 

Some of the current Free Swimming Burial ROVs can interface to a tracked work 

package. This provides the Free Swimming Burial ROV the opportunity to have 

a stable work platform for burial operations and to revert to free swimming 

mode when inspection and intervention tasks are required as well as more 

manoeuvrability, especially in the deep water operations. Figures 3.20 and 3.21 

show the CM ROV3 cable burial vehicle. This vehicle can operate either in free 

swimming mode or can have a modular tracked unit fitted (as shown in the 

Figures). Figure 3.21 shows the jetting lances in the fully deployed condition. 

Some Free Swimming Burial ROVs now have power budgets of over 300kW and 

are equipped with manipulators for handling tasks, together with cable cutters, 

cable grippers and burial tools fitted to both the forward and rear sections of the 

ROV. In addition jetting lances, which are fitted to the end of a manipulator arm, 

allow for localised burial. 

Dredging units fitted to Free Swimming Burial ROVs have a particular use when 

working far offshore in deeper waters, where seabeds can be encountered which 

consist of shell beds held together by fine sands. These seabeds are particularly 

difficult to trench using jetting systems, as the energy from the water jets deflects 

off the hard shell surfaces. This can be countered by using a combination of 

dredging and jetting as the dredging sucks in the sand and shells and breaks up 

the layer composition. 

51



Another generation of dredging only units have occasionally been used for 

cable burial work at inshore locations. These dredging units are large suction 

machines which are generally used to excavate large holes in the seabed and 

are more commonly used for seabed levelling and dredging activities. Owing 

to the imprecise nature of their mode of operation, scarcity of use, and because 

they have a significant environmental impact, they are considered unlikely to be 

used for the burial of offshore wind farm cables. They have not, therefore, been 

included in the scope of review of this study. 

Figure 3.20: CM ROV3 

(Photo Courtesy of CTC Marine Projects) 

52

Figure 3.21: CT2 Launch 

(Photo courtesy of CTC Marine Projects) 


53



Table 3.12: Free Swimming Cable Burial Machines 

54 

Type Burial Systems 

Available 

FREE SWIMMING CABLE BURIAL MACHINES 

Maximum Burial 

Capability 

Atlas 1 Jetting System Up to 1.0m in sands 

and low to medium 


Atlas 2 Jetting System Up to 1.0m in sands 



Excalibur Jetting System Up to 3.0m in sands 



Scorpion C/N 

Burial Sled 

Various Work 

Class ROVs with 

Burial Sleds 

Various Work 

Class ROVs with 

Burial Sleds 

Jetting System Up to 1.0m in sands 









CT 1 Jetting System Up to 3.0m in sands 



CMROV 1 Jetting System Up to 1.0m in soft 

soils only 

CMROV 2 Jetting System Up to 1.0m in soft 

soils only 

CMROV 3 Jetting system Up to 1.5m in sands 



CMROV 4 Jetting system Up to 1.5m in sands 



SCARAB III Jetting System Up to 1.0m in soft 

soils only 

Scarab IV Jetting System Up to 1.0m in sands 

and clays 

Triton ST200 Jetting System Up to 1.0m in sands 

and low strength clays 

Maximum 

depth of 

Operation 

Operator Manufacturer 

2000m Global Marine Perry Slingsby 

Systems 


Systems 


Systems 

3000m Acergy – 

formerly 

Stolt Offshore 

Saipem / 

Sonsub 

Canyon 

Offshore 


Projects Ltd 


Projects Ltd 


Projects Ltd 


Projects Ltd 


Projects Ltd 

1000m – 

burial 

2000m – 

inspection 

1000m – 

burial 

2000m – 

inspection 

2500m Tyco 

Submarine Ltd 


Systems 

Sonsub 

- 

- 

SMD 

Hydrovision 

Ltd 

SMD 

Hydrovision 

Ltd 

SMD 

Hydrovision 

Ltd 

SMD 

Hydrovision 

Ltd 

ACMA Group Oceaneering 

Ltd 

ACMA Group Perry Slingsby 

Systems 


Systems

3.8.5 BURIAL SLEDS 


Most burial sleds are developed for the burial of the shore end section of cable 

systems and work in shallow water. As well as being used for open water shore 

end cable installation, these machines are often used for river crossing and 

estuary cable work. 

Most of the systems are relatively simple in their technology and consist of a 

basic frame structure to provide a pathway for the cable and a simple hydraulic 

function to allow for a jetting tool to be deployed to the required depth of cable 

burial. In most instances the water pumps for the jetting tools are provided from 

the host vessel with pipework direct to the burial sled. These low cost shallow 

water burial tools inevitably require diver intervention for cable loading and 

unloading. 

Burial sleds are also used in shallow water areas where the seabed or intertidal 

zone consists of very soft muds or where seabeds will not provide any bearing 

support to a tracked cable burial vehicle or a conventional subsea plough. 

The burial sled is likely to be a lot lighter than these types of burial machines, 

particularly if the burial systems utilise surface powered jetting systems which 

negates heavy subsea pumps and motors. It can be possible to make these 

burial sleds almost neutrally buoyant by the attachment of temporary buoyancy 

and diver lift bags. Hence, the lightweight configuration allows the burial 

sleds to work in soft seabeds which would be out of the range of other burial 

machines. 

A selection of typical burial sleds which are currently used worldwide are 

listed in Table 3.13. The same table also includes details of injector systems. 

Injectors are large specialised jetting tools which are typically used in harbours 

and anchoring areas where deep burial is required (up to 10m). These units are 

powered from the surface and use massive pumps and power packs to deliver a 

very potent force into the seabed to achieve the depth of burial required. 

55



Table 3.13: Burial Sleds 

56 

BURIAL SLEDS 

Type Burial Systems Available Maximum Burial Capability Operator Manufacturer 

Bantam Jetting System 2.2m Global Marine 

Systems Ltd 

Injector Deep Water Jetting Tool – 

Specialised Application 

Between 2.0m and 10.0m Global Marine 

Systems Ltd 

Panzer Jetting System Between 2.0m and 5.0m Global Marine 

Systems Ltd 

Sabre Surface Powered Jetting 

Inductor Tool 

2.0m Global Marine 

Systems Ltd 

ETA Ltd 

Unknown 

Unknown 

Unknown 

TJV 06 Jetting System 2.0m Travocean Unknown 

TJV 05 Jetting System Up to 4.0m Travocean Unknown 

TJV 04 Jetting System Up to 2.0m Travocean Unknown 

TM7 Jetting System Up to 3.0m Land and Marine Unknown 

3.8.6 ALTERNATIVE CABLE PROTECTION METHODOLOGIES 

When cable protection cannot be achieved by cable burial, or for operational 

reasons cable burial is not the preferred method for cable protection, there are a 

number of other alternative cable protection methodologies available to ensure 

subsea cables are protected from hostile seabed intervention. 

The alternative cable protection methodologies which are reviewed in this 

section are employed on a worldwide basis and are suitable for all offshore wind 

farm development projects. 

Typical examples where alternative cable protection methodologies would be 

utilised are as follows: 

● 

● 

● 

● 

● 

● 

Sections of cable close to J-tubes where the cable burial machine commences 

burial at a transition point away from the J-tube (typically 10m to 20m in 

distance); 

Areas along the cable route where the subsea cables cross existing cables or 

pipelines; 

Areas of surface laid cable which has resulted from the need to recover a 

subsea cable plough due to weather or other operational reasons; 

Where burial has been difficult to achieve and sections of cable have been 

surface laid in the original cable burial operation; 

Where for operational reasons it has not been practical to mobilise a cable 

burial machine; and 

Environmentally sensitive coastal and sea cliff areas where directional drilling 

is employed.


Each alternative cable protection methodology will have its relative merits and 

detractions and the selection of the most appropriate form of cable protection 

will be dictated by the requirements at a particular location. 

Concrete mattresses 

Concrete mattresses are widely used on power cable and pipeline installations 

both as a means of primary cable protection and also for crossings over other 

existing subsea cables and pipelines. Concrete mattresses are prefabricated 

and consist of a number of concrete block sections connected by polypropylene 

rope. They are usually approved by other stakeholders, particularly fishermen 

who consider concrete mattresses to be potentially less damaging to their 

fishing gear than rock dumping. Figure 3.22 shows a concrete mattress being 

lifted and Figure 3.23 shows how a concrete mattress is deployed over a cable 

or pipeline. 

Figure 3.22: Concrete Mattress being Lifted by a Lifting Frame 

(Photo courtesy of FoundOcean Ltd) 

57



Figure 3.23: Concrete Mattress being Deployed Over a Pipe Section 

(Photos courtesy of FoundOcean Ltd) 

58


The installation of concrete mattresses requires diver intervention to assist in 

their accurate placement. However, for safety reasons most mattresses are 

deployed with automatic release systems connected to the rigging, with divers 

either out of the water or well clear when each mattress is released. It is normal 

practice for a number of concrete mattresses to be transported to the work site 

on a host vessel with its own vessel crane which is used to individually deploy 

the mattresses. Figure 3.24 shows the deployment of a concrete mattress. 

Figure 3.24: Concrete Mattress being 

deployed by Crane from a Host Vessel 

(Photo courtesy of FoundOcean Ltd) 

Concrete mattresses can each weigh up to 10 tonnes and a typical mattress 

would measure 5m x 3m x 300mm thickness. However, smaller and lighter 

mattresses are also available. The size, weight and density of the concrete used 

will be dictated by the stability calculations which have to be undertaken in 

advance of the mattress deployment. Such calculations will take into account 

any potential scour effects around the periphery of the mattresses. 

Where concrete mattresses are used for cable or pipeline crossings, the procedure 

would involve the placement of concrete mattresses over the existing pipeline 

with the new cable then laid on top of these mattresses with a further layer of 

mattresses then added over the new cable to create a sandwich effect. This 

approach ensures satisfactory separation between the new cable and existing 

cable or pipeline whilst also ensuring a robust cable protection solution for the 

crossing arrangement. This procedure is illustrated in Figure 3.25. 

59



Figure 3.25: Typical Detail for the Cable Crossing Arrangement using 

Concrete Mattresses to Provide Separation and Cover 

Rock dumping 

Rock dumping is an established methodology for protecting subsea cables both 

along their installed lengths and also at crossings with existing subsea cables 

or pipelines. 

Many of the major offshore rock dumping contractors such as Van Oord, 

claim the technique is very resistant to both trawling and anchoring activities. 

However, rock dumping is not generally favoured by fishermen who claim it sets 

up a potential snagging point for their fishing gear. Furthermore, certain groups 

see the introduction of substrates which are foreign to the locality as falling 

into the category of undesirable. However, there is a counter review from other 

groups who view the introduction of a new rock reef as a potential new habitat 

for fish and other underwater species to colonise. 

In shallower waters rock dumping is usually achieved either by the use of side 

tipping vessels which deposit their cargo in a somewhat crude method that often 

results in the need for larger volumes of rock to achieve the primary protection 

method or by using a grab device which is illustrated in Figure 3.26. 

60

Figure 3.26: Placement of Rock for Scour Protection 

Grout bags or sand bags 


Grout or sand bags can be regarded as a smaller scale version of the mattressing 

process. They are usually installed by divers to stabilise or fix in place a cable or 

a pipeline over short distances. 

Grout bags can either be deployed as pre-filled bags or for larger applications 

empty fabric bags are taken to the seabed and a diver coordinates the filling of 

the grout bag using a grout mix and pumping spread from the host vessel above. 

Figure 3.27 shows a Grout Bag (in an onshore trial) which has been placed over 

a pipe section and filled with grout. 

Figure 3.27: Grout Bag over a Pipe Section and Filled with Grout 

61



Frond mattresses 

Frond mattresses can potentially increase protective cover over a surface laid 

section of cable by stimulating the deposition of sediments which are suspended 

and transported in the water column. When the sediments connect with the 

frond mattresses, they are forced to settle; this eventually forms a new sand 

bank. Before a frond mattress is selected for a particular application it would be 

essential to assess the appropriateness of the technique for the local conditions 

and it may be necessary to deploy a frond mattress at the site in advance of 

the cable protection project to ensure that the procedure would be successful. 

Figure 3.28 illustrates a frond mattress. 

Figure 3.28: Frond Mattress Protection 

Figure 3.29 illustrates how a frond mattress and a concrete mattress could 

be used in conjunction to protect cables and pipelines close to an offshore 

structure. 

Figure 3.29: Protection using a Frond Mattress 

in conjunction with a Concrete Mattress 

62


Fronds are popular with environmental organisations as there is a perception 

that the indigenous substrate is being reinstated. However, experience has 

shown that situations such as storms, which create high energy situations on the 

seabed, have resulted in deposited materials around the fronds being stripped 

out. 

The installation procedure for the frond mattresses would be identical to that 

explained above for concrete mattresses. 

Uraduct 

Uraduct is manufactured using high performance polyurethane elastomer. The 

system comprises two cylindrical half shells which overlap and interlock to 

form close fitting protection around the core product such as cable, umbilical 

or flexible/rigid flowline. For ease of handling, the half shells are manufactured 

in lengths of up to 2.0 metres with flexing characteristics to suit the required 

minimum bend radius of the product or ancillary shipboard lay equipment. 

The half shells are secured in place using corrosion resistant banding located 

in recessed grooves to ensure a smooth external profile. A range of sizes is 

available to support product outside diameters ranging from 15mm to 450mm 

and beyond. 

Uraduct comes in varying sizes and stiffnesses to resist different levels of impact 

and is of particular use at cable crossing points or in areas close to structures, such 

as offshore wind turbine support structures or offshore oil and gas installations, 

where the risk from dropped objects is high. Uraduct is not commonly used 

along longer sections of cable which has been inadvertently surface laid on the 

seabed as it does not offer any further protection from trawling gear or anchors 

and only increases the target size. 

Articulated metal shell connectors 

Articulated metal shell connectors are typically used to provide cable sections 

with added mass and abrasion resistance in high energy environments such as 

a cable shore landings, which pass over rock outcrops and where other forms 

of cable burial are not possible. The articulated sections would be applied by 

surface divers in half sections which are then locked or bolted together to form 

a continuous pipe section. 

Directional drilling 

Directional drilling is employed by cable installation contractors when the 

topography of a landfall site makes it difficult to achieve a conventional landfall 

by trenching, or where there are environmental interest features that need to 

be avoided such as saltmarsh or chalk cliff. Directional drilling provides a very 

useful alternative, as it allows the cable installation to bypass the critical areas 

63



and only causes localised disruption at the drill site which can be reinstated 

when the directional drilling equipment leaves the site. 

Figure 3.30 shows a typical directional drilling procedure. This procedure has 

been extracted from the LMR Drilling website (http://www.Imrdrilling.co.uk/intro. 

html). The directional drilling process can be undertaken in four separate phases 

and is described in detail below. 

Drilling the profile. A small diameter pilot hole is drilled under directional control 

to a predetermined path using a mud-motor or jet bit on the end of the pilot 

string. The pilot string is drilled up to 80 metres in length, then the washover 

pipe is advanced in rotary mode until it is approximately 30 metres behind the 

drill bit. Alternate pilot string and drilling operations take place until the exit 

point is reached. Then the smaller pilot string is removed. 

Enlarging the hole. Pre-reaming operations are carried out to enlarge the drilled 

hole to a size suitable for accepting the product pipe. Pull-back pipe is added 

behind the reamer. Depending upon the pipe diameter to be installed several 

pre-reaming operations may be necessary, each progressively enlarging the 

hole. 

Installing the pipe. The pull-back pipe is connected to a ‘cleaning’ reamer which 

in turn connects to a swivel joint, (to prevent pipe rotation) that is attached to 

the pipeline towhead. The drill rig is then used to pull the product pipe into the 

preformed hole. The drilling fluid consisting of water and clay minerals will 

remain in the annulus and protect the pipe. 

Figure 3.30: Typical Directional Drilling Procedure (http://www. 

Imrdrilling.co.uk/intro.html) 

64

Site set up 


The directional drilling rig is set up on an appropriate landward site and the 

directional drill is commenced from the land site position to the target point 

on the shoreline. The target point on the shore would be the end point of the 

directional drill and the entry point for the subsea cable into the conduit which 

would be inserted into the drilled hole. For most directional drill sites where a 

duct is being installed to receive cables from an offshore wind farm, the footprint 

of the land site would typically be 30m x 30m in area. 

Drilling procedure 

The procedure basically consists of drilling out to the target position and then 

pulling a duct pipe back through the drilled hole. 

65



A small diameter pilot hole is drilled under directional control along a 

predetermined path using a mud-motor or jet bit on the end of the pilot string. 

The pilot string is drilled up to 80 metres in length, before the washover pipe 

is advanced in rotary mode until it is approximately 30 metres behind the drill 

bit. Alternate pilot string and drilling operations take place until the exit point 

is reached. The smaller pilot string is then removed. If necessary, pre-reaming 

operations are carried out with pull back pipe added behind the reamer to 

enlarge the drilled hole to a size suitable for accepting the duct pipe. 

A pull-back pipe is connected to a ‘cleaning’ reamer which in turn connects to a 

swivel joint, (to prevent pipe rotation) that is attached to a pipeline towhead. The 

drill rig is then used to pull the duct pipe into the pre-drilled hole. Drilling fluid 

is used to lubricate the process which consists of water and clay minerals which 

will remain in the annulus and protect the pipe. 

Cable pulling and jointing 

After the drilling procedure has installed a duct for the cable path, the cable can 

be pulled up through the duct and connected in a joint transition pit. 

For an offshore wind farm, the export cables would be laid up to the exit point 

of the duct on the beach and connected to the pulling line which would have 

been pre-installed through the duct created by the directional drilling. The line 

would then be used to pull the cable through the duct and a mobile winch unit is 

required to provide the pull force. When the cable end reaches the joint transition 

pit it is strain connected (to safeguard the future integrity of the cable joint) to 

a fixed point in the joint transition pit and the cable joint is made between the 

subsea cable and the land section of cable. 

Site restoration 

Following the completion of the drilling and cable pulling operations the land 

site would be restored and the joint transition pit would be secured with locks 

on the entry point to the pit with a security fence around the joint transition pit. 

Access to the joint transition pit is retained for the operational life of an offshore 

wind farm to secure a maintenance capability on the joint. 

3.8.7 CABLE BURIAL IN MOBILE SEABEDS 

The burial of subsea cables in areas where the seabed sediments are mobile has 

always posed a significant technical challenge to the cable installation industry 

to ensure that the cables remain fully protected for their design operational 

life. 

If a subsea cable is buried along the longitudinal line of a mobile seabed area, 

such as a sandwave area, there is a high probability that sections of the cable may 

66


become exposed as a consequence of moving seabed sediments. Experience 

gained from the post installation surveys of subsea telecommunications cables 

has shown that sections of cable which may have been left surface laid during 

the initial installation have subsequently been re-buried by the movement of 

sandwaves, whilst at the same time previously buried sections of cable have 

been exposed. 

In order to mitigate the effects of mobile sediments, deeper cable burial is often 

specified. This solution may work where the amplitude of the sandwaves are 

relatively low (typical 1m to 1.5m). However in areas where the amplitudes are 

much higher, deeper burial will not be possible on a practical basis. Furthermore, 

if deeper burial is attempted in these areas there would be a potential for a 

significant impact on the local environment. 

To successfully mitigate the effects of mobile sediment seabeds, the only 

effective method is to undertake accurate and detailed surveys and to carefully 

plan the cable route to avoid any potential problems. The survey data should 

provide detailed bathymetric, geophysical and geotechnical information and the 

results of this survey should then be reviewed against historical chart data to 

ascertain any evidence of the mobility of the seabed in the area where the cables 

are planned to be buried. 

With all survey and historical data to hand, a detailed cable route risk assessment 

can be completed. This will identify the optimum route for the cables and may 

well involve a complete route diversion to avoid mobile sediments such as 

sandbanks etc. The subsea cables can also be routed to follow the troughs of any 

major sandwaves or sandbanks such that any movement of sands would result 

in mobile sediments being deposited over the cables as opposed to exposing 

them. However, future accessibility of the cables for any potential repair or 

maintenance activities needs to be considered if such a policy is adopted. 

3.8.8 CROSSING NAVIGATION CHANNELS 

When the planned cable routing for the export cables requires a crossing of 

either an existing navigable channel or a proposed route for a navigable channel, 

it is necessary to install the cables to a suitable reference elevation to protect 

the cables from any future planned dredging operations along the line of the 

channel. There are two options which are commonly employed. 

Option 1 – the pre-excavation method 

The pre-excavation method requires an initial pre-excavation across the 

navigable channel before the export cables are installed. Export cables would 

then be buried using a subsea cable plough (or possibly a trenching vehicle) 

which will pass over the recently pre-excavated channel, during the cable lay 

procedure. It is anticipated that the cable crossing corridor across the channel 

67



would be up to 50m wide per cable and this section of the channel would be the 

only area subject to local excavation. 

Excavation would either be achieved by use of conventional grab dredging 

equipment or by using specialised remotely controlled underwater excavation 

machines which are used for localised excavation. Figure 3.31 (Option 1) shows 

the profile of the anticipated locally excavated section. In the example provided, 

the local excavation would have the objective of lowering the seabed level from 

reference Elevation +8.5m down to reference Elevation -10.5m. 

Following the completion of the local excavation, a subsea cable plough or 

trenching vehicle would then traverse across the locally excavated section 

cutting the 1m depth of trench and simultaneously burying the cable to this 

target depth during the cable lay procedure. This will then ensure a target cable 

installation to reference Elevation -11.5m. 

Option 2 – deep burial with plough or trenching machine 

In order to avoid any pre-excavation across the navigable channel it may be 

possible to utilise a subsea plough or trenching vehicle fitted with either a deep 

burial plough share or cutting tool. The burial machine would be deployed to 

the full penetration depth of 3m when the plough or trenching unit traverses 

across the navigable channel. The same burial machine would then revert to the 

standard target depth of burial for the remainder of the export cable route. 

Figure 3.31 (Option 2) shows the concept of deeper burial either using a subsea 

plough or the trenching vehicle. 

The likelihood of success of this methodology would be dependent upon the 

local geotechnical conditions which exist at the point of the crossing of the 

navigable channel. These conditions would be verified by local site investigation 

prior to any installation activities commencing. 

68


Figure 3.31: Methods to Achieve Cable Installation to Reference 

Elevation -11.5m across the Navigable Channel 

3.8.9 BLASTING THROUGH ROCK 

Blasting through rock has not been included in this review as an alternative 

cable protection methodology as it would be discounted on the grounds of high 

cost and significant impact to the environment. Where rock was encountered at 

a cable landing zone, horizontal directional drilling would be the preferred cable 

protection method. Further offshore, either a rock ripping plough, rock wheel 

cutter or a vibratory share plough would be used. 

69



3.9 Cable Protection for Offshore Wind Farm Developments 

This section of the report focuses specifically on the various cable protection 

methods which are relevant to the offshore wind farm industry. This section also 

includes feedback from cable installation works which have been completed on 

a number of UK Round 1 offshore wind farms. 

Cable burial will be the primary cable protection methodology for the export 

cables which connect the wind farm to the shore and for the inter-array cables 

which connect the offshore WTGs to each other and inter-arrays. Section 3.9.1 

reviews the cable burial methodologies which were presented in Section 3.8 and 

notes which of these techniques are most relevant for offshore wind farms. The 

type of cable burial machine which is used on any offshore wind farm project 

will always be determined by the seabed conditions which exist at the site, the 

choice of contractor who is selected to undertake the installation work and the 

availability of appropriate cable burial machines. 

All of the alternative cable protection methodologies which were described 

in Section 3.8.6 could potentially be employed on offshore wind farm subsea 

cables. 

3.9.1 CABLE BURIAL OPTIONS FOR OFFSHORE WIND FARMS 

Within Section 3.8, four separate types of cable burial machine were identified, 

namely: 

● 

● 

● 

● 

70 

Cable burial ploughs; 

Tracked cable burial machines; 

Free swimming ROVs with cable burial capability; and 

Burial sleds. 

Table 3.14 provides a summary of each of the above in terms of their relevance 

and potential application for the burial of offshore wind farm subsea cables. 

Table 3.15 provides further guidance for each burial device (with the optional 

configurations of varying burial tools on such devices) on what performance 

each option is likely to achieve in various seabed sediments. It must be stressed 

that this qualitative approach is somewhat subjective as within each defined 

burial device category there will be varying performances linked to the power 

capability of any given device.


Table 3.14: Particular Relevance for Cable Burial Machines for 

Offshore Wind Farms 

Cable Burial Machine Burial Machine Options Relevance for Offshore Wind Farms 

Cable Burial Ploughs Cable ploughs are available in 

varying types: 

● Conventional narrow share cable 

ploughs 

● Advanced cable ploughs 

● Modular cable ploughs 

● Rock ripping ploughs 

● Vibrating share ploughs 

Tracked Cable Burial 

Machines 

Free Swimming 

ROVs with Cable 

Burial Capability 

Tracked cable burial machines can 

be equipped with the following 

burial tools: 





Free swimming ROVs can be 

equipped with the following burial 

tools: 



Burial Sleds Burial sleds can be equipped with 

the following burial tools: 





All types of cable burial ploughs could be 

employed for the burial of cables on offshore 

wind farms. The type of plough employed will 

be determined by the nature of the seabed 

conditions. It is more likely that the cable ploughs 

will be used on the export cable routes as the 

lengths of cable are more suited to the burial 

method. 

Tracked cable burial machines could be used 

for the burial of the export cables, but are more 

likely to be employed for the burial of the interarray 

cables for offshore wind farms. The type of 

tracked cable burial machine will be determined 

by the nature of the seabed conditions. Those 

fitted with jetting systems only will be limited 

to work in sandy and soft clay seabeds whilst 

those with mechanical cutting tools will have an 

extended range of use. 

It is unlikely that free swimming ROVs equipped 

with a cable burial capability would be widely 

used for the burial of subsea cables on offshore 

wind farms. These machines are more commonly 

used in deeper water applications and when an 

intervention task such as a repair is required. 

Burial sleds are typically used for shallow water 

work and usually require diver assistance for the 

loading and unloading of cable. They may be 

utilised for the shore section of wind farm export 

cables but are unlikely to be used extensively for 

all cable systems for offshore wind farms as they 

tend to be less productive and responsive than 

their tracked cable burial machine counterparts. 

71



Table 3.15: Guidance on the performance of burial devices 

(with burial device options) in various seabed sediments 

Cable Burial Devices Burial Device Options Sediment Type 

72 

Sands Silts Gravel Weak 

Clays 

Stiff 

Clays 

Cable Burial Ploughs Conventional narrow 

share cable ploughs 

Tracked Cable Burial 

Devices 

Free Swimming ROVs 

with Cable Burial 

Capability 

Rock 

Advanced cable ploughs 

Modular cable ploughs 

Rock ripping ploughs 

Vibrating share ploughs 

Jetting systems ? 

Rock wheel cutters P P P 

Chain excavators P P 

Dredging systems ? ? 

Jetting systems ? 


Burial Sleds Jetting systems ? 

KEY 

 

= Should be capable of burial. 

Rock wheel cutters P P P 

Chain excavators P P 


? = Performance will be related to the type of sediment and the power delivery to the burial device. 

P = Performance possible in the sediment type but not an ideal application. 

 

= Unlikely to be capable of burial. 

Cable burial for offshore wind farms is usually undertaken using a barge which 

is specifically mobilised with cable handling and burial equipment. The same 

barge will also be equipped with a number of heavy duty tow winches which 

are capable of deploying a six or eight point anchor system to provide stability 

during cable burial operations. An anchor handling vessel is independently 

used to lift and place anchors in order to allow the cable installation barge to 

appropriately reposition itself as the burial works progress. 

In circumstances where the cable installation barge is the host vessel for cable 

burial operation using a towed subsea plough, the anchors are deployed in such a


manner which allows the barge to tow the plough whilst hauling on the deployed 

anchor array until such time as the slack is taken up on the anchor wires and the 

anchors have to be repositioned. The frequency of change of anchor positions 

will be dependent on a number of factors such as water depth, predicted tow 

forces and the size of barge etc. However, as a typical guide on a number of wind 

farm projects undertaken to date, a burial length of between 100m and 200m 

has typically been achieved on a single set of anchor deployments before the 

anchors have to be repositioned. The deployment of anchors must be carefully 

monitored and controlled whilst in the vicinity of other cable or pipeline systems 

as would typically occur at a pipeline or cable crossing location to avoid any 

potential damage to such existing infrastructure. 

3.9.2 CABLE CORRIDORS FOR EXPORT CABLES 

It is standard practice for the export cables from an offshore wind farm to 

be consented to be installed within a cable corridor. The cable corridor may 

typically be 500m to 600m wide of the centreline of the planned cable route and 

provides the wind farm developer and cable installation contractor a degree of 

flexibility to cope with the following: 

● 

● 

● 

Avoidance of any localised obstructions such as wrecks etc. which do not 

appear on Admiralty Charts but are discovered in later surveys specific to the 

project; 

Localised areas of difficult ground conditions where burial may prove difficult. 

For example small rock outcrops, boulder formations or glacial till outcrops; 

and 

Routing of cables within the corridor limits to follow the troughs of sandwaves 

in areas where mobile seabeds are known to exist. 

3.9.3 CABLE INSTALLATION ON OFFSHORE WIND FARMS TO DATE 

This section of the report contains cable installation information for offshore 

wind farms which have been installed to date. It should be noted that only 

limited information was available for a number of sites listed in Table 3.16. 

Supplementary information has been obtained from additional news items and 

data published on websites and with reference to articles which have appeared 

in the technical press. Therefore, the assembled data is a collection of market 

information from a wide variety of sources. 

73



Table 3.16: Cable Installation Experience and Feedback on Recently 

Installed Offshore Wind Farms 

Wind Farm Cable Route Soil Conditions Burial Depth and 

Method 

Arklow Bank Export Cables Superficial 

sediments 

overlying harder 

sediments which 

are believed 

to possibly be 

glacial till. 

Nysted 

(Denmark) 

74 

Export Cables 


Inter-Array 

Cables 

Horns Rev Inter-Array 

Cables 

It is believed 

there are a 

variety of seabed 

conditions across 

the site. 

A subsea cable plough 

was used to bury the 

export cables. 

A variety of installation 

techniques were 

employed. Jetting was 

used in areas of looser 

substrates which included 

sands, silts and clays with 

shear stress less than 

75kPa. Pre-trenching and 

backfilling was used to cut 

through areas of harder 

substrate with back-hoe 

excavators working from 

a shallow water jack-up 

barge. 

Feedback Information 

(where available) 

It is understood that surveys 

undertaken post installation 

indicated localised exposures 

of cables. This was possibly 

caused by scouring of 

the superficial sediments 

adjacent to the bank. It is also 

understood that the export 

cable suffered a fault resulting 

from an anchor contact with 

a repair being completed 

within one week. Further cable 

protection methods are under 

consideration. 

It is understood that the 

cable installation and burial 

were successfully completed. 

However, the burial operations 

took a considerable period of 

time owing to poor weather. 

The offshore spread does 

not appear to be particularly 

weather resistant potentially 

which explains the apparently 

protracted operations. 

- - It is understood that the 

installation contractor 

encountered difficulties 

whilst installing and burying 

the inter-array cables. It is 

thought that the wave climate 

proved problematic during the 

installation operations and it 

is believed that divers were 

subsequently employed to 

bury the cables which were left 

exposed, particularly close to 

the offshore structures.


Method 

North Hoyle Export Cables Close to the 

wind farm there 

were stiff clays 

associated 

with glacial till. 

However, the 

vast majority 

of the route 

consisted of 

sands and 

gravels. 

Inter-Array 

Cables 

The array on 

the east side of 

the wind farm 

had stiff clays 

associated with 

glacial till. The 

remainder of the 

site consisted 

of sands and 

gravels. 

A Sea Stallion subsea 

cable plough with a 2m 

burial depth capability 

was employed to achieve 

a target burial depth of 

1.5m. 

The LBT1 trencher was 

used which simultaneously 

laid and buried the interarray 

sections of cable to a 

target burial depth of 1m. 




The installation work 

proceeded well without any 

problems. One of cables was 

installed from the shore to the 

wind farm and the other cable 

installed from the wind farm to 

the shore. Both cables crossed 

the BHP owned pipeline to 

the Hamilton offshore facility. 

Concrete mattresses were 

used to separate and protect 

the cables at the crossing 

and diver intervention was 

required to bury the sections 

of cable from the edge of the 

mattresses to the termination 

points of ploughing 

operations. 

The LBT1 is equipped with a 

forward mechanical cutting 

tool which sits in front of the 

main jetting unit for cable 

burial. It achieved good burial 

across the majority of the site 

apart from when working in 

the stiff clays on the eastern 

array where burial was 

only possible in the softer 

sediments. The LBT1 also has 

to overrun the cable line to 

allow the final cable end to 

be pulled in the J-tube. This 

typically leaves a 50m section 

of cable unburied close to 

the second WTG structure. 

Diver techniques were used to 

bury these sections of cable 

and this resulted in the burial 

works being carried over into 

a second season following 

the main installation of the 

offshore wind farm. 

75




Method 

Scroby 

Sands 

76 

Export Cables Mostly sands 

along the export 

cable route. 

Inter-Array 

Cables 

Mostly sands 

along the interarray 

cable route. 

Burial was undertaken 

using a Sea Stallion 

plough which had been 

modified to achieve a 

target depth of burial of 

3m. 

Burial was undertaken 

using a Sea Stallion 

plough which had been 

modified to achieve a 

target depth of burial of 

3m. 

Kentish Flats Export Cables - The export cables were 

buried using the Global 

Marine owned Hi-Plough. 

Inter-Array 

Cables 

- The inter-array cables 

were buried using the 

Global Marine Otter 

tracked vehicle in jetting 

mode. 



It is understood that all three 

export cables were generally 

buried to the target depth of 

burial of 3m and this deep 

burial requirement was a 

permit requirement based 

on EMF and its effect on 

migratory fish as well as 

concerns with mobile sands. 

The second export cable 

had to be cut during the 

installation due to adverse 

weather, an offshore joint was 

subsequently introduced and 

the repair splice buried with 

post lay burial techniques. 

It is understood that some 

inter-array cables have become 

exposed as a consequence 

of scour around the base of 

the offshore structures. It is 

also believed that remedial 

rock dumping has been 

undertaken but the results of 

this subsequent intervention 

are unknown. 

The Hi-Plough was fitted with 

jetting equipment to reduce 

tow forces during installation. 

However, the jetting system 

was not utilised, partly owing 

to concerns with respect to 

suspended sediments affecting 

local shell fish beds and fish 

spawning grounds. Sediment 

load downstream of plough 

operations was monitored and 

the limits imposed by CEFAS 

for increase in sediment load 

over background levels were 

not breached at any time 

during operations. 

Difficulties in burial operation 

were encountered where 

the cable installation had 

crossed spud depressions 

from the main installation 

vessel – Mayflower Resolution. 

Generally the work appears 

to have been successfully 

completed without any major 

incidents.


Method 

Barrow Wind 

Farm 

Export Cables Gravels and 

glacial tills. 




Subsea cable plough. A subsea cable plough was 

used to bury both the export 

cables. During the installation 

of one of the cables an 

operational incident occurred 

in which the plough overran 

and damaged the cable which 

resulted in the need for an 

offshore joint. 

3.10 Burial Assessment and General Survey Techniques 

Burial assessment survey (BAS) techniques are commonly employed by the 

subsea telecommunications industry. A BAS survey is conducted along the 

proposed corridor for the installation of the subsea cable to provide an advance 

indication of the likely seabed conditions which will be encountered during cable 

burial operations. The survey will also give further confirmation of the seabed 

topography. 

The BAS survey will be complementary to the other surveys which are 

commonly employed along the potential planned cable route. These surveys 

would comprise: 

● 

● 

Geophysical survey – to establish the bathymetry and seabed profile and also 

identify any potential hazards along the proposed cable route. 

Geotechnical survey – this will usually consist of vibrocores and cone 

penetration tests (CPT) along the cable route to establish the viability of cable 

burial. Boreholes are not usually undertaken as they are expensive and the 

cable installer is only interested in data for the top 2m to 3m of seabed and 

the CPT and vibrocore data adequately provides this data. 

The BAS survey is usually undertaken using a burial assessment survey tool 

which is normally a scaled down simplified version of a narrow share cable 

plough. The BAS tool is pulled along the route of the proposed cable well in 

advance of the scheduled cable installation work as part of the pre-survey 

operations. The BAS tool provides real time data obtained for the penetration of 

the tool into the seabed. This includes the towing resistance of the tool, pitch and 

roll data, plus sonar and visual data if camera systems are fitted to the tool. 

In certain cases the burial assessment tool can be a simple grappling hook, 

although the feedback results are not as precise as those obtained from the 

plough-like BAS tool. The data is still meaningful in that it provides an indication 

of the potential difficulties in cutting a trench into the seabed. All BAS tools have 

the added advantage of clearing the route of any unchartered debris such as 

disused or out of service cables or discarded anchor chain or wire. This creates 

77



scars on the seabed, the persistence of which will depend on the local seabed 

conditions. In coarse sands and gravels the grappling hooks will penetrate 

to a depth of up to 1m and infilling of the resulting scar will occur almost 

instantaneously (EMU Ltd, 2004). In coarser grounds, the grappling scars will be 

more persistent; cobbles will be displaced and overturned. 

The data from the burial assessment survey is then used to generate a burial 

protection index (BPI) for the complete cable route. The BPI takes into account all of 

the potential hazards to the cable system and assesses the results from all survey 

data, including the BAS survey. The BPI is then used to design the type of armouring 

along the cable route, i.e. rock armour, double armour or single armour and to derive 

a burial depth which will take into account the safety of the cable system whilst 

ensuring an environmentally friendly and cost effective installation. 

By assessing the BPI correctly for the complete route of the cable, projects 

can expect routes with a varying burial depth requirement. This may result 

in burial depths varying from 0.5m to 2.0m along the route. The subsea 

telecommunications industry (and the cable installation contractors working 

within this industry) have established that it is far better to only bury cables 

to the minimum depth required rather than comply with a global target burial 

for the complete system length. A significant number of the cable installation 

contractors who work in the offshore wind farm industry have experience with 

the subsea telecommunications industry and they are lobbying to have a transfer 

of technology so that the lessons learnt can be applied for offshore wind farms 

in UK waters. The argument which states why try to embed a cable down to 

3.0m if the cable is safe and has been buried with an acceptable burial method 

to 1.0m is compelling and deserving of serious consideration. 

There is a real possibility that BAS surveys may become part of the survey regime 

for offshore wind farms. Therefore, the environmental impacts of a BAS survey 

may need to be considered as part of future consenting requirements. However, 

as the BAS tool will generally be a scaled down version of the primary burial 

tool intended for the actual burial works, there are no different environmental 

effects other than those already described for the primary burial tools. The only 

difference is that the BAS tool will make a first pass along the route in advance 

of the primary burial tool completing the installation of the subsea cable. 

3.11 Decommissioning 

The Energy Act has not yet provided any clear guidance on the legislation related 

to the decommissioning of offshore wind farms. However, it is almost certain that 

all of the offshore structures would have to be removed to the seabed (partial 

removal). The nacelle and towers would be removed in reverse operation to 

construction using a heavy life vessel. Cables would be disconnected after being 

isolated offshore and pulled out of the J-tubes. 

78


It is probable that the subsea cables would be left buried and notified as being 

disused or out of service. The reason being that to de-bury the installed subsea 

cables is likely to result in a significant disturbance to the seabed. If cables are 

only buried to a shallow depth in sandy seabed, an under-runner can be used 

to de-bury the cable. This device is put on the cable and as the name suggests 

‘under runs’ the cable while being towed from a line from a host vessel. This 

procedure, however will not work for deeper buried cables as is commonly 

associated with offshore wind farms (1.0m and deeper), especially when the 

cables are effectively buried in clays, gravels, chalk etc. Therefore, de-burial 

would involve the use of significant subsea plant (as yet not commercially 

available to the market on a significant scale) using aggressive methods such as 

cutting large open trenches to access the buried cables. 

Certain sections of cable would be removed for a decommissioned offshore wind 

farm. This would include both the beach section of cable down to the low water 

point and the sections of cables close to the offshore WTGs before full depth of 

burial is achieved away from the J-tubes on the structures. It is difficult to predict 

if cables would be fully removed at any disused cable or pipeline crossing. The 

crossing point is likely to consist of concrete mattresses or similar. Over the 

course of time this construction is likely to have formed an artificial reef; to 

remove the crossing construction would therefore have a negative impact on the 

subsea environment local to the crossing point. 

79

4 Physical change 


The purpose of this section is to describe and, as far as is possible, quantify 

the fate of sediment disturbed when the cable burial techniques described in 

Section 3 are employed in a range of varying seabed conditions. It addresses the 

physical nature of the change but not the potential effect on parameters which 

is covered in Section 5. 

The key parameters that will affect the volume of seabed sediment disturbed, 

brought into suspension, dispersed in the surrounding sea area and finally 

re-deposited on the seabed during cable burial operations are: 

● 

● 

80 

The cable technique being used; and 

The site conditions i.e. seabed type, tidal and wave conditions. 

This appreciation of the extent of the physical disturbance resulting from cable 

burial operations enables the environmental impacts discussed in Section 5 to 

be placed in context. 

The main parts of this section cover: 

● Section 4.2 outlines the site conditions that can influence the type of 

equipment necessary for cable burial. 

● Section 4.3 discusses the level of sediment disturbance that is likely to occur 

when cable burial is undertaken with the range of techniques discussed in 

Section 3 and the ground conditions defined in Section 4.2. 

● Section 4.5 discusses the dispersal and re-deposition or settlement of 

sediment brought into suspension. This section also covers methods available 

to quantify the dispersal and settlement and discusses recent experience 

from the UK offshore wind farm industry. 

4.2 Site Conditions 

4.2.1 SEABED CONDITIONS 

The geology and geomorphological characteristics of the seabed will determine 

the type of sediment disturbed during the cable burial operations and will also 

have a significant influence on the type of equipment used for these operations. A 

good understanding of seabed conditions is, therefore, required for engineering 

planning and to assess the environmental impacts of the burial operations. 

The ground conditions that will be encountered during cable burial may 

comprise a range of material sizes from fine to coarse sediments and rock. 

To aid understanding and provide consistency, soil and rock types discussed

Physical change 

throughout the report are defined in Table 4.1. This is based on the soil 

classification produced by the Permanent International Association of Navigation 

Congresses (PIANC) and reproduced in BS 6349, Part 5. 

Table 4.1: Classification of ground conditions 

Type Particle Size and Characteristics 

Clays Cohesive 

Less than 0.002mm 

Exhibits strong cohesion and plasticity 

Can vary from Very Soft (shear strength less than 20kN/m2) to Hard (shear strength 

greater than 150kN/m2) 

Silts Can range in size from 0.002mm (fine) to 0.06mm (coarse) 

Some cohesive strength, non plastic or low plasticity 

Sands Can range in size from 0.06mm (fine) to 2mm (coarse) 

Cohesionless 

Can vary in strength between loose, dense and cemented 

Gravels Can range in size from 2mm (fine) to 60mm (coarse) 

Cohesionless 

Strength generally loose but possible to find cemented gravels 

Boulders/cobbles Can range in size from 60mm to 200mm (coarse) 

Cohesionless 

Strength loose 

Rock Sedimentary (including Chalk), Igneous, Metamorphic 

Can vary from Very Weak (Compressive strength ,200MN/m2) 

Structured or unstructured/structureless (See Section 4.3) 

4.2.2 TIDES 

An understanding of the tidal conditions at the site is essential to assess the 

fate of sediment brought into suspension during cable burial operations. Tidal 

flows will transport suspended sediment away from the cable route. The time 

the sediment will remain in suspension is largely determined by the particle 

size, with coarse sediments (sand and gravels) re-depositing on the seabed 

relatively quickly and finer sediments (silts, clays and chalk particles) remaining 

in suspension for a greater length of time. The extent of the seabed over which 

the sediment is re-deposited is largely determined by the strength of the tidal 

currents. The stronger the tidal currents, the greater the excursion of a suspended 

sediment particle and, therefore, the extent of the seabed that will be subject to 

re-deposition. However, with sediment dispersed over a larger footprint than on 

more moderate tidal flows, the depth of deposited sediment is likely to be less. 

4.2.3 WAVES 

Wave conditions may affect the fate of suspended sediment. Their influence is, 

however, likely to be secondary to the tidal conditions as water depths along 

the majority of the cable route are likely to be such that tidal processes are the 

primary forces driving sediment movement. Wave data will primarily be required 

81



for the planning of the offshore cable installation works rather than to assess the 

fate of disturbed sediment. 

4.3 Level of Sediment Disturbance 

The level to which the seabed is disturbed is primarily related to the nature of the 

ground and the type of tool selected to bury the cable. The type of device used 

to support the tool is likely to have a secondary influence. 

This section discusses the application of the cable burial tools described in 

Section 3 in the range of ground conditions described in Section 4.2. The 

following cable burial tools have been considered: 

● 

● 

● 

● 

● 

82 

Ploughs Simple 

Advanced Jetting 

Deep Burial 

Rock Ripping 

Vibrating 

Jetting Fluidisation 

Erosion 

Dredging 

Rock Wheel Cutter 

Mechanical Chain Excavator 

The section covers the likely effect of these systems in terms of the level of 

disturbance to the seabed. Section 4.3.5 attempts to quantify this disturbance by 

ranking the effects of the cable burial systems and provides limited advice on the 

calculation of the volume of sediment disturbed and brought into suspension. 

4.3.1 PLOUGHING 

Cable ploughs (Section 3.8.2) are generally used in sand, silt, all types of clay 

and weak rock such as structureless chalk, which generally consists of sand to 

cobble-sized fragments in a matrix of silt. They can also be used in harder rock 

if the plough is fitted with a rock penetrating tooth, or in chalk and gravel beds 

if a vibrating plough share is used. 

The controlled operation by which cable ploughs work, “displacing” the 

sediment into which the cable is lowered, followed by the natural backfilling 

of the trench ensures that soil disturbance is kept to a minimum, and also 

minimises mixing between soil particles and the surrounding water. The type of 

soil most susceptible to mixing with water during ploughing is silt, because silt 

possesses no internal cohesion and the particles are small enough to be eroded 

by gentle water turbulence. Silt may remain in suspension for days giving the 

current chance to transport the sediment some distance away from the trench.


Structureless chalk, as for silty soils, is also susceptible to mixing and dispersion 

in the surrounding sea water during ploughing. 

4.3.2 JETTING SYSTEMS 

Jetting systems are one of the most commonly employed cable burial tools 

used by tracked vehicles (Section 3.8.3), ROVs (Section 3.8.4), and burial sleds 

(Section 3.8.5). The mechanisms employed by jetting systems for developing a 

trench will largely depend on the soil type. 

Cohesionless soils 

In cohesionless soils, the soil can be liquefied or fluidised with jets at relatively 

low pressures (i.e. low jet exit velocities). Liquefaction or fluidisation occurs 

when the pore water pressures in the soil become equal to the total overburden 

stresses, reducing the effective stresses to zero. In both cases, the soil particles 

and water behave very much like a dense fluid. However, there is a distinct 

difference in the two conditions. In liquefaction, the volume and bulk density 

is more or less constant (Ishihara, 1993). In fluidisation, the water content is 

increased and the soil structure is completely broken down to give a material of 

lower density. An increase in the jet flow rate at low jet pressures simply causes 

an increase in the volume of the liquefied/fluidised soil (N.B. an increase in flow 

rate at constant pressure can only be achieved by increasing the total nozzle 

area). If the jet pressure/velocity is increased, the velocity of the fluid flowing 

over the surface of the soil will increase until eventually the soil particles are 

lifted off and transported away from the soil mass as suspended sediment. This 

is the process of erosion or scour. 

A trench in cohesionless soil is thus created by a process of erosion/scour. 

Jetting systems are sometimes used in combination with a dredging system 

to increase the rate of removal of soil (see Section 4.3.3). The problem with 

liquefaction in cohesionless soil is that the trench walls collapse and flow back 

into the trench. This means that a lot of soil has to be removed before there is a 

significant increase in trench depth. The final trench shape in cohesionless soils 

tends to have very gentle sloping sides (Lincoln, 1985). It may require several 

passes before a trench is created with sufficient depth. 

Due to the difficulty in forming a trench in cohesionless soils, some jetting 

systems do not even attempt to create one. The “fluidisation train” used for 

burying offshore pipelines in cohesionless soils is one such example (Boom, 

1976). The train works by fluidising the soil beneath a pipe so that the pipe can 

sink under the combined weight of itself and the train. Water is injected into the 

soil around the pipe at relatively low pressure, which is just enough to cause 

fluidisation but not necessarily erosion. 

The likely effects will depend on which mechanisms take place. If a trench is 

formed by erosion, a substantial amount of material may be transported away 

83



from the trench area as suspended sediment. The extent to which the sediment 

will be spread is dependent on the current velocity and the particle size. For 

example, coarse sand and gravel will settle out on the seabed very close the 

trench, especially in a slack current. In contrast, silt and fine sand will remain 

in suspension longer, and could be transported for a significant distance in a 

strong current. 

If the jetting system only fluidises the soil to allow the cable to sink through it, 

the impact will be negligible, since there will be no sediment displacement. 

Cohesive soils 

Liquefaction/fluidisation is a principal mechanism in the breaking up of 

cohesionless soils during water jetting because the pore pressures in the soil 

mass are able to respond relatively quickly to the increase in water pressures 

caused by the jets. In contrast, in cohesive soils (clays), the response is relatively 

slow, by which time the soil has been broken up by a different mechanism. 

In cohesive soils, the general erosive potential is much reduced due to the 

cohesive bonds between particles (Dunn, 1959; Flaxman, 1963). However, 

localised erosion and scour enables the jets to begin to form cuts in the solid 

material. Each jet tends to cut its own slot based on the various directions of 

the jets. The rate at which this cutting process takes place depends on the soil 

strength and the jet velocity/pressure at the cutting face (Lincoln, 1985). However, 

in very soft clays the jets may erode soil in a similar mechanism to that for sand. 

As the jetting proceeds, the high water pressures in the cut slots rapidly cause 

hydraulic fractures to develop (Hubbert & Willis, 1957; Jaworski et al., 1981; 

Murdoch, 1993). These develop along planes of weakness and have a tendency 

to propagate perpendicular to the direction of minimum principal stress. 

Propagation may therefore be expected in both vertical and horizontal directions 

according to the depth of the jet slot. It is probable that most propagation will be 

in the vertical plane causing cracking and failure of the forward face. 

In stiff or hard clays, certain jetting systems utilise a number of high pressure 

jets which are used to cut sections of the clay face. These are combined with 

lower pressure jets on the same jetting tool to then ‘break up’ the fractured clay 

face. 

As the jetting system advances, the inward pointing jets, particularly at the base, 

will further break up the already broken lumps of soil and finalise the cutting of a 

complete trench, provided the progress is sufficiently slow. If an attempt is made 

to advance the system too quickly, the jetting tool will come into contact or near 

contact with the soil face and the tow force will significantly increase. 

Normally, a dredging system needs to be used in combination with the jetting 

system in firm to hard clays, otherwise it is very difficult to remove the cut 

material away to form a trench. However, even when a dredging system is 

84


also used, some weakened material and lumps of disturbed soil will remain on 

the bottom of the trench to receive the pipe or cable as it is lowered into the 

trench by the forward movement of the burial device. The depth of pipe or cable 

sinkage will depend on the size, number and remoulded shear strength of the 

soil lumps. 

Although the trench will be wider than the jet tubes, due to outward pointing 

jets, this will be limited by the smaller separation normally required between the 

jetting system and the cutting face in cohesive soils. In many instances where a 

high power jetting system has been deployed in a hard soil seabed the trench 

width will be close to the spacing of the jet tubes. The trench sides will stand 

vertically for all but the very softest clays, although it will be broken and irregular 

due to the jet slotting action and the effects of hydraulic fracture. 

In very soft to soft clays, much of the soil will be eroded and mixed with the 

surrounding seawater. Clay sized particles will remain in suspension much longer 

than sand or silt sized particles, and could be transported long distances by the 

prevailing current. However, this means that when the sediment is eventually 

re-deposited on the seabed, the thickness of deposition will be very small. 

In firm to hard clays, most of the soil will be broken into sizeable lumps before it 

can be completely eroded. The lumps would need to be removed by a dredging 

system in order to form a trench. Even in strong currents, the heavy weight of 

the lumps will ensure that they are deposited close to the trench. This means 

that the thickness of deposition may be large, but the extent only very limited 

and localised adjacent to the trench. 

Weak rocks (including chalk) 

Only jetting systems which have a very high power delivery will have any impact 

on weak and fractured rock seabed. It is very rare for a subsea jetting system to 

be deployed to attempt to cut trenches into a rocky seabed. 

The mechanisms for forming a trench with a jetting system in weak rocks will 

be a combination of those that take place in cohesive and cohesionless soils. In 

structured rock, the mechanisms will be very similar to those in hard cohesive 

soil, with fluidisation being non-existent, erosion very localised, and rock breakup 

being dominated by hydraulic fracturing. In unstructured rock that easily 

breaks down into small hard fragments, both erosion and hydraulic fracture will 

dominate, with fluidisation contributing where the fragments are small. 

The mechanisms for chalk will depend on the grade and density of the material. 

Structureless Grade D chalk will probably behave like a cohesionless material 

with the silt matrix becoming easily suspended in water by the processes of 

erosion. Since structureless chalk typically has a large range of particle sizes 

(from silt-size to cobble-size), it may not be possible to fluidise the material 

sufficiently to enable a cable to sink without losing a substantial proportion of 

the finer material to erosion. Structured Grade A chalk would behave like a hard 

85



clay, with hydraulic fracture occurring very easily along existing fracture joints. 

However, it would be difficult to break the chalk into smaller blocks than already 

exist in situ, especially in medium to high density chalks. A dredging system 

used in combination with the jetting system would have difficulties in removing 

large rock fragments to create a trench. 

In most weak rocks including structured medium to high density chalk, other 

trenching systems will be more suitable, such as a plough with rock penetrating 

tooth or a rock wheel cutter. 

In structured weak rocks, the material would be mainly fractured/cut into 

sizeable blocks before much of the material could be eroded or fluidised. In this 

case, any trench would have to be formed by a dredging system, which would 

lift the blocks and deposit them adjacent to the trench. In structured soft, finegrained 

rocks, such as chalk, there will also be some disintegration of the rock 

into its constituent components, and this material could become suspended in 

the surrounding water and transported by the current some distance from the 

trench. However, as mentioned earlier, jetting is highly unlikely to be used for 

trenching in structured rocks. 

In structureless rocks, the material would be broken down into a wide range 

of particle sizes from clay or silt-size up to boulder-size. The clay to fine sandsize 

particles will become easily suspended in the water and transported long 

distances by the current. The larger sizes, that could only be removed using a 

dredging system, would be deposited close to the trench. As such, the impact 

will be a combination of the impacts for cohesionless and cohesive soils. 

4.3.3 DREDGING SYSTEMS 

Dredging systems used on tracked cable burial machines (Section 3.8.3), ROVs 

(Section 3.8.4) and burial sleds (Section 3.8.5) can be used for a range of soil 

types including sand, silt and certain clays. However, the larger the particle 

size, the harder it is to move material in a slurry suspension. Dredging systems 

remove soil to create a trench by a process of suction. The suction tubes are 

sometimes referred to as eductors. The educted soil is normally “blown” into 

the surrounding sea to the sides of the trench, where it is also transported as 

suspended sediment by the current until it settles out on the seabed. Dredging 

systems work best when the soil is in a slurry state. For this reason they are 

normally used in combination with a jetting system that is used to fluidise the 

soil prior to dredging. 

Dredging systems can remove the soil material and deposit it on a barge or, 

more conveniently, disperse the sediment into the sea away from the trench. 

The dispersal distance and thickness of deposition will depend on the particle 

size and current speed, in the same way as for eroded materials. The impact that 

the dispersed sediment makes on the local aquatic environment will depend on 

how well adapted the marine life is to this type of sediment deposition. 

86

4.3.4 ROCK WHEEL CUTTERS AND CHAIN EXCAVATORS 


In hard clays and rock, mechanical chain excavators or rock wheel cutters (Section 

3.8.3) are often used to create a cable trench. In these materials, a narrow slot 

is formed into which the cable is lowered. The action of cutting the rock or hard 

clay causes the material to be broken down into its constituent components, 

such as sand for sandstone, and silt for siltstone, limestone or chalk. In order to 

form an open slot the loose material has to be removed. This material naturally 

mixes with and becomes suspended in the surrounding seawater. Due to the 

protection provided by the hard ground, the slot does not need to be very deep, 

typically only 0.5-1.0m. 

Chain excavators are also sometimes used in sands and gravels. The movement 

of the chain fluidises the granular soil in the vicinity of the cutter, forming a low 

resistance “slot” for the cable to be pushed through. In this case, because the 

soil is being fluidised without an open slot being formed, the disturbed material 

can and does largely remain contained within the ground. Thus, the amount of 

sediment becoming dispersed is minimal. 

The effect of using chain excavators or rock wheel cutters in hard clay or rock is 

to disperse clay to sand sized particles into the surrounding seawater. However, 

the mass of the suspended sediment will be limited by the size of the cut slot. 

This suspended sediment will be transported away from the trench to some 

distance dependent on the particle size and current velocity. 

4.3.5 QUANTIFICATION OF DISTURBANCE 

Ranking disturbance 

An attempt has been made to rank the level of seabed disturbance resulting 

from cable burial operations discussed above. The results of this exercise are 

presented in Tables 4.2 and 4.3 where a ranking from 1 to 10 has been adopted 

with 1 indicating a low level of disturbance and 10 a high level of disturbance. 

Inevitably the tables present a simplification of a complex process but, 

nevertheless, provide a broad indication of the level of disturbance that is likely 

to occur when cabling burial operations take place in a range of seabed types. It 

is also worth noting that other seabed interventions such as offshore dredging 

or certain forms of aggressive fishing which filter the upper layer of seabed 

would have significantly higher rankings (see Table 4.4). 

87



Table 4.2: Level of sediment disturbance arising from use of plough 

types in different ground conditions 

88 

Plough Type Ground Conditions 

Conventional Narrow 

Blade 

Sand Silts Gravels Clay Unstructured 

Rock – Matrix 

Material 

Weak Rock 

(Chalk) 

1 1 1 1 N/A N/A 

Advanced with Jetting 2 3 2 2 2 N/A 

Deep Burial 1 1 1 1 1 N/A 

Rock Ripping 1 1 1 1 1 4 

Vibrating 1 2 1 1 2 6 

Level of disturbance 1 = Low, 10 = High, N/A = Not applicable 

Table 4.3: Level of sediment disturbance arising from use of other 

burial tools in different ground conditions 

Tool Ground Conditions 

Jetting Fluidisation 

Erosion 

Sand Silts Gravels Clay Unstructured 

Rock 

Chalk Structured 

Rock 

2 2 N/A N/A N/A N/A N/A 

3 4 3 3 N/A N/A N/A 

Dredging 4 6 4 N/A N/A N/A N/A 

Rock Wheel 3 4 3 3 3 8 4 

Mechanical Chain 

Excavators 

Level of disturbance 1 = Low, 10 = High, N/A = Not applicable 

Quantification 

3 4 3 3 3 N/A N/A 

There is very limited research and advice on the quantification of the volume of 

material that is disturbed and brought into suspension by cable burial operations. 

In the absence of detailed information conservative assumptions will need to be 

made for the following parameters: 

● 

● 

● 

Volume of material disturbed; 

Rate of sediment release into the water column; and 

Vertical distribution of suspended material throughout the water column. 

An estimate of the rate at which sediment is disturbed can be made based on 

the size of the slot or trench created by the tool. For cutting tools the rate of 

sediment disturbed can be calculated based: 

Depth of Deployment of Tool x Tool Width x Rate of Progress


A similar approach may be adopted for a plough but it should be appreciated 

that these tools displace rather than remove sediment from the seabed. 

As an example, a wheel cutter tool 250mm wide forming a 1000mm deep trench 

at a rate of 250m/hour will cut 62m 3 /hr. Reviewing evidence from land trials of 

these tools and also reviewing subsea video a reasonable estimate of only 10 to 

15% of cut material will backfill into the trench with the remainder deposited at 

the sides of the cut trench or removed as suspended material. 

In the absence of more specific information, it is reasonable to assume all fine 

sediment (clays, silts and sands) disturbed during the cable burial operations 

would be brought into suspension. The distribution of this sediment throughout 

the water column will depend upon the size of the particles and level of 

disturbance caused by the cable burial system. Although coarser sediments are 

also likely to be brought into suspension this material will quickly settle back to 

the seabed and is unlikely to be dispersed by tidal currents. 

4.4 Seabed Disturbance by Other Activities 

When considering the potential impact of cable installation as a whole, it is 

necessary to put these into context with other influencing factors including 

natural perturbations such as storm activity and other seabed users such as oil 

and gas installations and aggregate extraction, both of which cause disturbance 

to the seabed. The main impact associated with cable installation relates to 

the physical disturbance of seabed and the subsequent creation of a sediment 

plume. These impacts are however localised in nature and are, in general, oneoff, 

short-term effects, with the seabed usually returning to its original state. 

When compared with the area affected by other activities, the spatial extent of 

cable installation is very small. Table 4.4 provides an indication of the extent of 

physical disturbance caused by other seabed uses in the UK continental shelf. At 

certain sites the cumulative and in-combination effects with other activities and 

projects will need to be considered during the assessment process in order to 

determine the overall effect on existing environmental parameters. 

89



Table 4.4: Examples of physical disturbance of the seabed by seabed 

uses 

Deep water 

90 

Activity Spatial Extent / 

Unit Area 

UK Offshore pipelines 5084km length of pipeline installed; giving total 

length of 5847km (assuming a 15% contingency for 

interlink pipelines) 

UK Oil and gas 

installations 

Shallow water 

UK Aggregate extraction 

(for 2005) 

UK Dredging disposal 

(for 2004) 

240 offshore platforms 

180,000 m 2 (assuming average plan area of 

30 m x 25 m x 240 offshore platforms) 

134 km 2 (area actually dredged) 

1257km 2 (licensed) 

21.2 m tonnes 

310 km 2 (licensed) 

30.1 m tonnes (wet tonnage disposed) 

4.5 Dispersal and Re-Deposition of Sediment 

4.5.1 METHODS 

Reference/Source 

Ongoing research by Bomel Ltd. 

on behalf of HSE 

Ongoing research by Bomel Ltd 

on behalf of HSE 

Crown Estate (2005) 

Cefas (pers. comm.) 

Once the sediment has been lifted into suspension it will disperse under the 

action of the tidal flows. The strength of these currents and the dispersion and 

settling characteristics of the suspended sediment will determine the footprint 

over which the sediment will be deposited. 

Sands and gravels disturbed during the cable burial operations will settle back to 

the seabed very rapidly and the footprint is unlikely to extend any great distance 

from the cable route. Silts, clay and chalk particles will remain in suspension 

for a greater period of time and will be dispersed over a much greater distance, 

depending upon the strength of the tidal currents. However, the depth of 

deposition over such a large area is likely to be small. 

Relatively simple dispersion and deposition calculations can be undertaken 

to assess the magnitude of the problem based on tidal flow data presented in 

the form of “tidal diamonds” on published Admiralty charts or measurements 

from the site. Alternatively more sophisticated methods may be adopted using 

sediment dispersion models to track the movement of sediment particles. Such 

models require tidal flow inputs from hydrodynamic models. 

When calculating the predicted physical changes it is important that the changes 

are put into context with natural and other anthropogenic changes that occur. 

The effect of the physical change on receptors within the footprint of change will 

be dependent on a number of factors including the ambient levels of suspended 

sediment and the degree of variation throughout the year, for example during 

storm events. If the natural levels of suspended sediment and the seasonal


variation are high then the degree of impact is likely to be less. There is 

considerable variation in the natural and anthropogenic induced levels of 

suspended sediment around the UK coast. Suspended sediment concentration 

varies around the UK from 1-327 mg/l around the English coast and 1-227 mg/l 

around the Welsh Coast but annual mean values are typically 1-110 mg/l (Parr 

et al., 1998; Cole et al., 1999). Other areas, particularly near estuaries will have 

higher concentrations. In the study area of the proposed Sheringham Shoal 

Offshore Wind Farm off the north Norfolk coast (Scira, 2006), concentrations 

varied from 10mg/l to 30mg/l and between 30mg/l (in summer) to 60mg/l (in 

winter) for the study area for Thanet offshore wind farm, off the east Kent coast 

(Royal Haskoning, 2005). The variability in the ambient levels of suspended 

sediment and the seasonal variation that can be experienced mean that it is 

not possible to state what the potential effect of cabling could be on certain 

receptors. 

In addition an area of seabed subject to disturbance already will be less affected 

by subsequent changes. In order to put the potential effects into context it should 

be considered against a number of other activities that occur within the marine 

environment, where studies have been completed to investigate potential and 

actual effects, e.g. aggregate extraction and fishing activity. Aggregate activity 

is a well controlled and monitored activity where levels of suspended sediment 

have been measured. There are a number of potential sources of increased 

suspended sediment concentrations including the release of material at the 

cutter or drag-head, overspill during hopper loading and sieving (which may 

be necessary in order to obtain an optimum sediment load). Of these potential 

sources the material suspended at the drag-head is the most likely to be similar 

to the material suspended during ploughing for cabling activities. Measurement 

of the plume generated by the movement of the drag-head alone have shown 

that the volume of sediment introduced into the water column is barely 

detectable and is in the order of 1% of the material introduced by screening 

and overflowing (Hitchcock et al., 1998, John et al., 2000). The scale of effect 

will obviously vary dependent on the sediment size and the relative amounts of 

overspilling/sieving undertaken but does provide a reasonable indication of the 

relative scale of effect when compared to aggregate extraction. It must also be 

borne in mind the temporary nature of the effect which is limited to one event 

(per cable) over a short time scale along with the immediate start of recovery of 

the seabed following disturbance. 

The low levels of sediment that are mobilised during cable laying mean that there 

will be only low levels of deposition around the cable route. The finer material will 

generally remain in suspension for longer but will settle and remobilise on each 

tide with no measurable material left in place. Coarser sediments are expected to 

settle within a few metres of the cable route and following disturbance are likely 

to recover rapidly, given similar communities in the vicinity. 

Case studies are provided below which illustrate some of the predicted physical 

changes resulting from cable laying activities. The resulting direct and indirect 

91



effects that could occur on the environmental characteristics are discussed in 

Section 5. Section 5 also puts the potential environmental effects into context 

with other activities within the marine environment. 

4.5.2 RECENT EXPERIENCE 

Introduction 

This section covers recent experience and case studies identified during the 

literature and data search completed for the study. As part of this search 

discussions were held with ABP Marine Environmental Research Ltd who, with 

Cefas and HR Wallingford, are undertaking RAG Sedimentation Theme Study 

SED01 – Review of Round 1 Sediment Process Monitoring Data – Lessons Learnt. 

The discussions were to ensure that any relevant data collected under SED01 

was made available to this study. 

Norfolk (Cromer) Offshore Wind Farm 

As part of the environmental impact assessment for the Norfolk (Cromer) 

Offshore Wind Farm (Norfolk Offshore Wind, 2002) Royal Haskoning undertook 

an assessment of the fate of sediment released during ploughing operations in 

superficial silts, sands and gravels and the underlying boulder clay and chalk. 

An assessment of the relative proportions of the fine and coarse sediments 

was made based on seabed samples and the depths to consolidated materials. 

Advice was provided by Engineering Technology Applications Ltd. on the volume 

of material displaced during ploughing and, conservatively, it was assumed all 

this material was brought into suspension. An allowance was also made for the 

volume of sediment disturbed by the plough skids. 

The re-deposition of the coarse sediment was assessed by considering settling 

velocities of the particles, tidal current speeds and the height of the release point. 

The calculations indicated that the footprint for the re-deposition of the coarse 

sediment was sensitive to both the tidal conditions (i.e. spring or neap) and 

the time of release within the tidal cycle. The results indicated that the largest 

deposition footprint but smallest depositional depth (200m either side of the 

cable with a deposition depth of a few millimetres) would occur with release at a 

mid spring tide. In contrast, the smallest footprint but greatest depth (20m either 

side of the cable with depth of approximately 10mm) occurred with release at 

high water neap (see Figures 4.1 and 4.2). For fine sediments it was considered 

that particles would disperse throughout the water column and background 

suspended sediments concentrations would only be raised by a few percent. 

92

Figure 4.1: Maximum Thickness of Sediment Re-Deposition 

(Norfolk Offshore Wind, 2002) 


93



Figure 4.2: Maximum Extent of Sediment Re-Deposition 

(Norfolk Offshore Wind, 2002) 

94

Sheringham Shoal Offshore Wind Farm 


A broadly similar approach was adopted by HR Wallingford, for the assessment 

of ploughing operations for the Sheringham Shoal Offshore Wind Farm. 

However, within the study a more sophisticated approach was adopted for the 

dispersion of fine sediment using the dispersion model SEDPLUME-RW. The 

model used the hydrodynamic output from a TELEMAC-2D flow model and the 

assumption of a logarithmic velocity profile through the water column to track 

the 3-dimensional movement of fine sediment particles (mobile surface layer 

comprising sandy gravel with less than 4% fines). The results indicated that 

dispersion of sediment was rapid with concentrations dropping to less than 

1mg/l above background within a single flood or ebb excursion. It was noted that 

dispersion occurred at a slower rate on a neap tide than a spring tide because 

of the lower rate of turbulent diffusion with footprints 4km and 9km either side 

of the cable route. The scenarios included ploughing through chalk, ploughing 

through a silt/clay/sand mix and trenching through chalk. 

For ploughing chalk during a neap tide, the dispersion footprint extends for 

around 9km in each direction, with concentrations dropping to levels of less than 

1mg/l (above background) within a single flood or ebb excursion. For the spring 

tide simulation the higher turbulence causes the chalk concentrations to drop 

below 1mg/l (above background) within 4km of the cable route. 

The results for fines arising from other bed types with high percentages of fines, 

the neap tide footprint extends less than 2km, while the spring tide footprint is 

very small. As before the neap tide footprint is larger due to the lower rate of 

turbulent diffusion. The extent of the footprint on both tides is less than that of 

chalk due to the lower amount of material available per metre length of cable 

and the settling of silt during periods of slacker flows (chalk is assumed to have 

zero settling). This result is applicable to much of the inter-turbine cabling within 

the wind farm site where there are no exposures of chalk. 

The volume of material released by trenching through chalk is much higher, 

and therefore the extent and persistence of concentrations above 1mg/l is much 

greater. The predicted plume extends more than 10km in either direction at a 

level of up to 20mg/l (above background) on a neap tide. The model predicts 

a gradual drift of the plume towards the shore over the six tides, but that the 

plume has dispersed to less than 1mg/l concentration before the end of the 

model run. 

The footprint of silt deposition was found to extend over a wide area, but at 

an undetectable rate. Even under slack water conditions, the maximum rate 

of deposition over the six tide simulation was less than 0.5mm in the areas of 

greatest deposition, and in most of the footprint area the rate was far less. This 

result is anticipated as the deposited fines will be re-suspended on each tide, 

with no measurable material left in place. 

95



Dispersion and deposition of coarser sediment was not modelled as sand 

will only be carried a few metres from the point of disturbance. The sediment 

distribution of the unconsolidated surface layer in large parts of the area affected 

by the cabling is predominantly gravely sand with a small amount of silt. 

The high sand/gravel content of the in situ sediment, together with the relatively 

small disturbance arising from cable ploughing or trenching to 1m depth, 

suggests that for most of the cable route the majority of any disturbed sediment 

will fall immediately to the bed in the immediate location of the cable. Because 

of the minimal disturbance, fine sand is almost all likely to remain within the 

bottom 1m -2m of the water column (even this is probably conservative) and 

typical settling velocities of around 10mm/s will ensure that the sand settles 

within half an hour (or less) or becomes part of the ambient near bed transport. 

Medium or coarse sand will settle within minutes. The vast majority of the 

disturbed sediment will initially resettle within 20m of the cable, with almost no 

sand being carried more than 100m from the cable except as part of the natural 

background transport. 

The presence of surface sand/gravel along much of the cable route restricts the 

extent of fine sediment dispersion, and the modelled results are only applicable 

to those limited areas where chalk or other competent beds are exposed or have 

only a very thin surface layer of mobile sand. The plotted model results are 

therefore conservative for a 1m depth of burial, but still show that suspended 

sediment will be quickly dispersed. In the areas where the export cables are 

proposed to be buried up to 3m (shoals / sand waves) the cable is installed in 

(mobile) sands only, with no disturbance of the underlying chalk or other beds. 

London Array Offshore Wind Farm 

An assessment of the cabling operations for the London Array Offshore Wind 

Farm has been undertaken by ABP Mer (RPS, 2005). The work assumed that 

jetting techniques would be adopted and burial depths would be between 1 and 

3m. An assessment of the surface sediments was based on a comprehensive 

grab sampling exercise supported by seismic and side-scan surveys. It was 

found that the inter-array and export routes were predominately covered with 

fine sands. In parallel with the assessment of seabed conditions, suspended 

sediment data was collated. This included data from the Southern North Sea 

Sediment Transport Study, archive data held at CEFAS and measurements taken 

specifically for the project. 

Limited modelling was undertaken to quantify the impacts of the cabling 

operations. An assessment was completed based on the likely type and volume 

of sediment that would be disturbed, an assessment of the fall velocity of the 

disturbed sediment and the ambient tidal flows that would carry suspended 

sediment. 

96


The assessment concluded that the fine sand disturbed during the cabling 

could typically be carried a distance of 1170m in the 30 minute period it would 

remain in suspension (based on peak flows). At other tidal conditions settlement 

would occur more rapidly and the distance fine sand would be carried would 

be significantly reduced. It was concluded that with the rapid dispersion of the 

sediment it was unlikely that concentrations would be measurable above the 

ambient conditions. Coarser sediments disturbed during the cabling operations 

would fall out of suspension in far shorter distances. 

The conclusions from the study were supported with reference to monitoring at 

Nysted and Kentish Flats Offshore Wind Farms. 

Nysted Offshore Wind Farm 

During the construction of the Nysted Offshore Wind Farm in Denmark, strict 

requirements were imposed on the release of sediment from sea bed operations 

including the installation of the array cable. To ensure compliance with the 

requirements monitoring was undertaken and subsequently reported upon by 

Seacon (2005). 

The wind farm is located in the Femer Belt separating Germany and Denmark 

and is approximately 2km from the coast in water depths between 6 and 9.5m. 

The surface sediments within the wind farm area are generally medium sands 

with very low silt/clay content. The thickness of the overlying sands varied 

between 0.5 and 3m, underlain with clay deposits. 

Cable laying operations were undertaken using jetting where the substrates 

permitted and using pre-trenching and backfilling where hard substrates 

were encountered. The trenching operations were undertaken using a back 

hoe dredger rather than the more specialist systems described earlier in this 

section. Measurements of turbidity were taken continuously during the cabling 

operations and daily mean and maximum values determined. The jetting 

operations resulted in significantly less turbidity than the pre-trenching and 

backfilling operations with mean and maximum values at 200m from the various 

operations of: 

Trenching Mean = 14 mg/l 

Max = 75 mg/l 

Backfilling Mean = 5mg/l 

Max = 35mg/l 

Jetting Mean = 2mg/l 

Max = 18mg/l 

These values compare with the restrictions set by the Danish Energy Agency of 

15 mg/l as a mean value and 45 mg/l as a maximum value. 

97



Seacon concluded that the higher rates of sediment release from the pretrenching 

and backfilling was a result of the larger volume of sea bed strata 

disturbed during these operations and the fact that the material disturbed during 

trenching was lifted to the surface for inspection. This meant that the sediment 

was carried through the full water column before being placed alongside the 

trench. Although the trenching was undertaken using non-specialist equipment, 

a comparison of the monitoring results for the trenching and jetting operations 

at Nysted support the disturbance levels set in Table 4.3 above. 

Kentish Flats Offshore Wind Farm 

For Kentish Flats, EMU Ltd undertook turbidity monitoring during the cable 

installation in fulfilment of the FEPA licence conditions (EMU Ltd., 2005). 

Background data was collected from October 2004 to the beginning of February 

2005. During the cable burial operations site measurements were taken 500m 

down-tide of the three export cables which were laid using ploughs. The results 

of the monitoring showed: 

● 

● 

98 

Marginal, short-term increases in background levels (approximately a 9% 

increase to the modal concentrations); and 

Peak concentrations occasionally reaching 140mg/l (equivalent to peaks in 

the natural concentrations driven by the tidal cycle). 

These site observations support the broad conclusions from the modelling 

undertaken for the Cromer, Sheringham and London Array wind farms and 

suggest that the environmental effects of cabling methods are likely to be shortterm 

and localised. 

Cable Burial Operations at Lewis Bay, Nantucket Sound 

In 2003 Applied Science Associates Inc undertook modelling simulations to 

estimate water column sediment concentration and sediment deposition 

resulting from the proposed embedment of submarine electricity cables in 

Lewis Bay, Nantucket Sound (Galagan et al., 2003). SSFATE model simulations 

were completed to quantify these impacts for cables buried to a depth of 6ft in 

sand-sized marine sediments. It was assumed that a jetting device would be 

used to create a trapezoidal trench measuring 6ft across at the top, 2ft across 

at the bottom and 8ft deep. It was also assumed that 30% of the total sediment 

fluidised within the trench would be evenly distributed vertically throughout 

the overlying water column with the remaining 70% remaining within the limits 

of the trench. The maximum flood and ebb tide currents of 0.6ft/s with a water 

depth of 3.5ft was adopted within the modelling. 

The modelling results indicate that sediment was re-deposited on the seabed 

within 200ft of the trench with a maximum depositional depth of around 25mm 

immediately adjacent to the cable route. The modelling also indicated that 

suspended sediment concentrations would reach a maximum value of 120 mg/l


along the cable route and reduce to less than 20 mg/l within 1000ft of the cable 

route. 

It is interesting to note that the results of the above modelling exercise completed 

in the USA are broadly in agreement with the modelling and site measurements 

discussed above for a number of wind farms in the UK. 

4.5.3 CONCLUSIONS AND FURTHER RESEARCH 

The findings from the studies undertaken for a number of UK offshore wind farms 

indicate that the disturbance to seabed sediments during cable burial operations 

are likely to be short term and relatively localised, particularly if ploughing 

techniques are employed. It is unlikely that cumulative or in-combination 

impacts will present significant problems because of the short term nature of 

the burial operations. 

As noted earlier there is limited research and advice on the quantification of 

material arising from cable burial operations, particularly if jetting, dredging or 

cutting/excavating methods are adopted. This lack of base information on the 

release of sediment into suspension limits the benefits that could arise from the 

use of more sophisticated modelling techniques to establish the fate of released 

sediment. It is, therefore, suggested that this is an area where further research 

would be of value. It is appreciated that monitoring requirements included in 

FEPA licences for UK Round 1 and 2 offshore wind farms will provide valuable 

data for the quantification of the impacts of cable burial operations. 

4.6 Mitigation Measures 

The previous sections have discussed the influence of cable burial operations 

on sediment disturbance. In practice the main areas where mitigation measures 

can be adopted to reduce this disturbance are during the selection of the cable 

route and cable burial method. Both the route and burial methods adopted will 

be a balance between the engineering issues associated with the route corridor 

(see Section 3.10 on the Burial Assessment Surveys) and the key environmental 

issues associated with the site. The range of environmental issues likely to be 

encountered is discussed in Section 5. 

An appropriate level of site investigation (see Section 4.2.1) is an essential to 

ensure that the optimum route and burial methods are selected for the cable. 

The level of data gathered should be sufficient to provide confidence in the 

selected route and burial method and allow the assessment of the potential 

environmental impacts. It is at this stage that mitigation (e.g. re-routing) may 

be considered should the environmental impacts be significant. The completion 

of an appropriate level of site investigation in the early stages of the project 

will reduce the risks of delays during the consenting process and the risk of 

major changes in the later stages of a project. It will also allow a more strategic 

99



approach to be adopted for cable route planning and avoid the need for reactive 

or “firefighting” measures to deal with environmental and engineering issues. 

Once the cable route and burial technique have been selected there are limited 

measures that can be adopted to reduce sediment disturbance. The precise 

timing of the works (e.g. over a spring or neap tide) and the speed at which 

the burial proceeds may have some influence over the sediment disturbance, 

however, these parameters are largely determined by operational constraints 

and it will generally not be possible to make significant changes in an attempt 

to reduce sediment disturbance. 

100

5 Potential impacts and 

mitigation measures 


This section addresses the potential environmental impacts that could be 

experienced during cable installation activities, together with possible mitigation 

measures that can be used to minimise the magnitude and significance of 

effects. Although the length of the corridor for cable installation impacts can be 

long, the footprint of impact is narrow, generally restricted to 2-3m width, with 

certain methods of installation requiring the use of anchors (see Section 3.9.1). 

The main impact associated with cable installation relates to the disturbance of 

sediment (Section 4.3) and the subsequent creation of a sediment plume. The 

disturbance is restricted to the area described above and the generation of a 

plume and subsequent settlement is localised, with the area affected dependent 

on geological and hydrodynamic characteristics. Although the scale of effect will 

be site specific it is important that the impact should be put into context with 

natural perturbations resulting from storm activity and other activities occurring 

in the vicinity including fisheries, aggregate extraction, and other sediment 

plume generating activities. 

Impacts and mitigation measures are described and discussed for: 

● 

● 

● 

● 

● 

● 

● 

● 

● 

Subtidal ecology; 

Intertidal habitats; 

Natural fish resource; 

Fisheries; 

Marine mammals; 

Ornithology; 

Shipping and navigation; 

Seascape and visual character; and 

Marine and coastal archaeology. 

Discussion of impacts is divided into two categories: 

● 

● 

Potentially significant impacts – which are considered relevant to the 

installation of cables and could result in significant environmental effects; 

and where relevant 

Other impacts – which may be considered to be impacts by stakeholders but, 

are unlikely to cause significant effects. 

101



Mitigation measures to avoid, reduce or minimise environmental effects have 

been cited along with a discussion on their effective implementation, where 

known. During the planning phases of a proposed offshore wind farm many of 

the potential impacts are reduced through good environmental management 

practices, including the avoidance of known sensitive sites or areas with high 

proportion of fines, chalk areas, sensitive areas for fish spawning, etc. These 

can occur where there is enough knowledge of the baseline environmental 

conditions. However, in other areas it is important that a degree of flexibility 

is present to enable certain mitigation measures to be applied, for example, 

seasonal restrictions and micro-siting of cable routes. 

5.2 Subtidal Ecology 

5.2.2 INTRODUCTION 

Subtidal habitats likely to be encountered by cable routes include the 

following: 

● 

● 

● 

● 

● 

● 

● 

102 

bedrock and boulders; 

reefs; 

chalk; 

gravel and shingle beds; 

sand and mud (silts and clays) banks; 

maerl beds; 

seagrass beds; and 

● biogenic reefs such as horse mussel beds (Modiolus modiolus), and Sabellaria 

spinulosa (Ross worm) reefs (Johnston et al., 2002). 

Potentially significant effects include: 

● 

● 

Seabed disturbance; and 

Increase in suspended sediment concentrations and subsequent settlement. 

Other effects include: 

● 

● 

● 

● 

Potential contaminant release; 

Electro magnetic effects; 

Heating effects; and 

Cable coating effects.

5.2.2 POTENTIALLY SIGNIFICANT EFFECTS 

Seabed disturbance 

Potential impacts and mitigration measures 

Some weeks prior to the installation of the cables, a cable burial assessment 

survey can be carried out (see previous Section 3.10). Disturbance to the seabed, 

leading to an alteration of habitat and associated species will occur during the 

burial assessment survey and during cable burial operations. Such impacts are 

likely to be limited to an area 2-3m either side of the cable, depending upon the 

size of the installation device used. In general, the range of installation devices 

available would have similar impact footprints, especially those that are tracked 

or have skids in contact with the seabed. Recovery of the seabed is, to some 

degree, dependent on the substrate that is left following installation. 

Certain techniques will aid recovery by infilling the trench following cable 

placement. In respect to this potential impact, the action of a narrow blade 

conventional plough is expected to fulfil this criterion more than any other 

method of installation. With a narrow blade conventional plough, a wedge of soil 

is lifted, the cable placed at the base of the trench and then the displaced soil 

wedge is allowed to backfill naturally. Any other excavation method will disturb 

the sediment and not allow layers of sediment to be reinstated in the same 

sequence in their natural state. For example, mechanical cutter or jetting systems 

have aggressive cutting mechanisms which are used to cut open trenches. 

Certain installation devices require the use of a vessel that utilises an anchor 

array to stabilise the cable laying vessel which would slightly increase the area 

of disturbance (see Section 3.9.1). As the vessel needs to move as the cable is 

laid, anchors are repositioned, increasing the area of potential impact. 

Where cable crossing (see Section 3.8.6) is necessary there would be a 

requirement for the placement of cable protection measures such as concrete 

mattresses, which would cover a larger area, in the order of 150m 2 for each cable 

crossing. The exact area would depend on the specified requirements from the 

existing cable owner. 

Installation of cables close to turbine foundations using either ploughs or 

remotely controlled tracked vehicles will always result in a short section of cable 

close to the J-tube exit points not being buried as a consequence of limitations 

in the cable burial procedure. A common solution for these short lengths of 

exposed cable (typically 10m to 15m) is to use either over covering concrete, 

frond mattresses or rock dumping. Rock dumping can be problematic, especially 

in and around sensitive habitats, as careful placement would be required in 

order to avoid unnecessary damage to habitats and species. Burial under the 

dumped rock would involve a permanent loss of habitat where the placement 

occurs. These rocks, similarly to all surfaces in contact with the sea, would be 

readily colonised by a range of fouling species, and may act to cause localised 

increases in biodiversity. However, artificial increases in biodiversity through the 

103



introduction of an essentially ‘alien’ substrate cannot be considered as a beneficial 

impact. As previously mentioned, another alternative is to use concrete or frond 

mattresses as a protective measure for cables. Frond mattresses will encourage 

the accumulation of sediment and, in an area that comprises soft sediments this 

is likely to be preferable as it is more in keeping with the natural environment. 

Mobile species, in the vicinity of the cable route and inter-array area, may avoid 

the footprint of the impact. Sessile species, however, would be damaged or killed 

during excavation through direct contact with the installation device, burial and 

dislodgement. The significance of the impact would depend on a number of 

factors including: 

● 

● 

● 

104 

The nature and geology of the seabed where the cable is to be laid. The 

geology needs to be ascertained to at least the depth of cabling to ensure 

that where sediments overlay bedrock within the depth of cabling activity, the 

effect of cabling through all habitat types is assessed; 

The nature of the assemblage of species within the footprint of the works (e.g. 

species abundance, richness and diversity in the study area, and the value 

of this assemblage in the context of the wider seabed area within which the 

cabling operation is to occur); and 

The sensitivity, importance and recoverability of the species/communities 

including any seasonal variation e.g. spawning activity and over-wintering 

species. 

In terms of the recovery of the seabed following disturbance, studies have shown 

that initial recolonisation takes place rapidly following a disturbance event with 

certain species returning almost immediately to the disturbed site. The length of 

time taken for recovery will be dependent on a number of characteristics including 

the nature of the seabed, the community types present, the duration and footprint 

of the proposed activity and the degree of disturbance already experienced at 

the site. There are a number of activities that can cause disturbance including 

severe storms, bottom fishing and aggregate extraction. Studies have shown 

that in shallow water and estuarine environments where disturbance is more 

frequent and opportunistic species are more likely to dominate the community 

structure, recovery occurs rapidly whereas in deeper water undisturbed areas 

the recovery to a more stable community could take many years. The degree 

of existing disturbance is therefore of importance in assessing this effect and in 

deeper water environments the impact of other activities, such as fishing, should 

be taken into consideration. 

Rates of recovery of invertebrate communities appear to be associated with the 

rate of recovery of the seabed sediment characteristics. Experiments undertaken 

to record recovery given different intensities of disturbance revealed that when 

sediment was removed to a depth of 10cm recovery of the faunal component 

occurred within 64 days of the disturbance. However, when sediment was 

removed to 20cm depth, recovery was not complete until after 107 days but had 

occurred within 208 days of the disturbance. Thus recovery at more intensely


disturbed sites took nearly twice as long. Nevertheless, the higher intensity 

disturbance did not have a significantly greater effect on the community than 

was found in the less intense disturbance (Dernie et al., 2003). This implies that 

cabling could take longer still for recovery due to the depths of disturbance. 

However, cabling activity replaces the sediment, albeit in a different structure, 

and the majority of the communities are within the top 10-20cm of the sediment 

indicating that recovery may be influenced strongly when disturbance intensity 

changes between these depths but may not differ too much once disturbance 

occurs below this depth. 

Studies have been undertaken in a number of habitat types in order to record 

recolonisation rates following dredging activity. The results of these studies 

are summarised in Newell et al., 1998, and show variation in recovery times of 

between 3 weeks for freshwater semi liquid mud and 12 years (and >7 years) 

for sand-gravels (and coral reefs). Studies have shown that adult migration has 

been observed as the major mode of recolonisation (Savidge & Taghon, 1988; 

Thrush et al., 1991). These recovery rates should however be put in context with 

the nature and extent of the disturbance compared to cable installation. Due to 

the localised nature of the cabling activity whereby the area affected is generally 

restricted to 2-3m width of substrate, the overall effect on the benthic ecology is 

not likely to be significant if the habitat distribution throughout the wider area 

is homogenous. However, if there are specific areas, directly in the path of the 

cable, where the habitats are not widely distributed and/or particularly sensitive 

to disturbance, then these will need to be avoided. One such example includes 

cable installation through areas of biogenic reef comprising of Sabellaria 

spinulosa, the reef building honeycomb worm. The reefs formed by the worm 

provide valuable habitat for many associated species and would be destroyed 

by cabling activity, albeit in very localised areas. They are also listed as a priority 

habitat under the EU Habitats Directive and specialised surveys are required 

in order to define the boundaries of the reef. Although these species have a 

high recoverability, their nature conservation importance means that any direct 

loss that damages the integrity of the reef structures or adversely affects their 

development would be considered as being of significance. 

In summary, potential effects from disturbance on seabed habitats are as 

follows: 

Rock – some scarring may occur dependent on the rock type e.g. effects on soft 

rock such as sandstone habitats will be more significant. Encrusting and attached 

fauna and flora can be dislodged/disturbed. Species inhabiting rock habitats are 

often sessile species and are therefore more susceptible to disturbance. 

Chalk – a permanent scar is likely. Cable burial techniques will disturb epifauna/ 

flora inhabiting chalk habitat. Disturbance of chalk will cause a high visibility 

plume which will remain in suspension for long periods of time, but which is 

unlikely to cause more than an aesthetic effect. 

105



Clay – A permanent scar will be left in stiff clay habitats following cabling 

activity. In soft clay, infilling is expected to occur rapidly. In harder or stiffer 

clays, a cutting wheel disc is often used which allows a wedge of soil to be cut by 

the action of the plough. This wedge is lifted by a ramp on the plough share, the 

cable is placed at the bottom of the trench and the wedge of soil then allowed 

to naturally backfill onto the cable. This process leaves minimum disturbance to 

the seabed with no spoil mounds. Spoil mounds are only found with ‘V’ shape 

plough shares more commonly associated with pipeline burial. Clay supports 

a species poor community due to the cohesive nature of the substrate. Cabling 

through soft clay is likely to put more sediment into suspension than in stiff clay 

where the habitat is more cohesive. 

Sand – Sand will infill rapidly following disturbance by ploughing or trenching. 

Burrowing species may be affected but are generally adapted to change through 

natural disturbance due to the mobility of the substrate. 

Gravel – Certain types of gravel habitat will infill immediately following cable 

laying activity, others may leave a shallow trough following initial infill. Generally, 

species inhabiting mobile gravel are adapted to harsh living conditions and 

would be expected to recover quickly. 

Suspended sediment 

Impacts resulting from cabling include the release of sediment into suspension 

(see Section 4.3). This can have a number of effects on the benthic species 

inhabiting areas adjacent to the cabling activity. The significance of the impact 

will be dependent on the type of sediment, the hydrodynamic conditions and the 

sensitivity of the species affected in addition to the type of installation method. 

The potential change in significance as a result of the type of sediment and 

hydrodynamic conditions is outlined in Section 4.3.5 and summarised in Tables 

4.2 and 4.3. 

Increases in suspended sediment can affect filtering mechanisms of certain 

species, such as specific types of worm and brittle stars, through the clogging 

of gills or damage to feeding structures. Suspended sediment can also attach 

to fish eggs causing abnormalities or death. The sensitivity of the receptor is an 

important consideration when determining the significance of this effect. This 

includes its tolerance to a given effect, but also to its potential for recovery from 

such an effect (discussed in sub-section above). Its tolerance will be similar in 

all areas of the UK but will also depend to some extent on its adaptability to 

its ambient conditions. If a species has adapted to survive in a wide variety 

of conditions (such as in estuaries where suspended sediment concentrations 

(SSC) may be measured in grammes per litre), it is more likely to survive a 

small increase in SSC than a species which is exposed to a lower variation in 

SSC throughout the year. Information on the sensitivity (tolerance) of species 

should be obtained from published literature and consultation with relevant 

106


experts. Information can be found on the Marine Life Information Network 

(MarLIN) website (http://www.marlin.ac.uk) and the UK Marine SAC Programme 

website (http://www.ukmarinesac.org.uk/marine-communities.htm) that provide 

detailed information on the sensitivity/intolerance of many key features of the 

marine environment. However, it should be noted that the knowledge base for 

determining sensitivity of species is limited and research is needed to increase the 

confidence in such predictions. Many of the species in the marine environment 

are likely to have some degree of tolerance to increases in suspended sediment 

in order for them to adapt to natural perturbations and are therefore likely to 

survive localised short term effects. 

A prolonged increase in suspended sediment concentrations can affect the 

penetration of light through the water column affecting the photosynthetic 

activity of macroalgae, phytoplankton and eel grass. This is unlikely, however, 

during the installation of cables, as suspended sediment is only a very localised 

short term effect. 

As the material settles out of suspension, it can cause smothering to sensitive 

species and can change certain habitat characteristics. Studies outlined in Section 

4.5.2 show the potential for settlement in site specific situations and generally 

conclude that the extent of the settlement will depend on the factors outlined 

above but, in general, the effect is expected to be short term and localised with 

one example showing settlement ranging from 20m up to a maximum of 200m 

either side of the cable, dependent on the state of the tide (Norfolk Offshore 

Wind, 2002). 

5.2.3 OTHER EFFECTS 

Potential contamination due to sediment disturbance 

Consideration must also be given to the potential for contaminant remobilisation 

during cable installation activities due to disturbance of contaminated sediments. 

It is, however, less likely that high levels of contamination would be encountered 

away from the coast, unless the cable passes close to a historic or active disposal 

site. The screening/scoping process will identify if the cable route is likely to 

encounter potentially contaminated sites; in which case it would normally be a 

requirement to undertake sediment analysis prior to decision making relating to 

this aspect. 

Electro magnetic field generation 

Submarine power cables can generate electro magnetic fields (EMF) in the 

surrounding seabed and water. The potential impact of EMF on fisheries is 

discussed in Section 5.4. 

107



It is currently unknown which invertebrate species could be affected but 

magnetic sensitivity has been demonstrated for the following: Decapoda 

(Crangon crangon), Isopoda (Idotea baltica) and Amphipoda (Talorchestia 

martensii and Talitrus saltator) (Greater Gabbard Offshore Winds Ltd., 2005). In 

all cases, magnetic sensitivity is understood to be associated with orientation 

and direction finding ability such that the animal may become disorientated; 

depending on the magnitude and persistence of the confounding magnetic field 

the impact could be a trivial temporary change in swimming direction or a more 

serious impact on migration (Greater Gabbard Offshore Winds Ltd., 2005). 

Although there has been no targeted monitoring specifically to investigate 

whether distributions of crustaceans and molluscs have been affected by the 

presence of submarine power cables and associated magnetic fields, monitoring 

to meet other specific objectives relating to offshore wind farms has not revealed 

any evidence to show such an effect. There are therefore uncertainties regarding 

the significance of this potential impact. However, it is not expected that the 

impact would be of significance, since the species that could be affected are 

known to be mobile and, as such, are able to avoid impacted areas. Generally 

the habitats that they inhabit are widespread and the effects of magnetic fields 

are usually highly localised around the cable. 

Potential heating effects 

The effect of radiated heat from cables buried in the seabed has been considered 

by the Connecticut Siting Council (CSC, 2001) as part of the ‘Cross Sound Cable 

Interconnector’ project, a high voltage DC buried cable system between New 

England and Long Island New York. The CSC estimated a rise in temperature 

at the seabed immediately above the buried cable of 0.19 o C and an associated 

increase in seawater temperature of 0.000006 o C. 

The potential rise in temperature is therefore considered to be impossible to 

detect against natural fluctuations in the surrounding sediments. 

Cable coating effects 

The leaching of chemicals and substances from cable coatings and cable 

sheaths are likely to have a minimal impact on the surrounding environment. 

Burial of the cable will further reduce possible environmental effects. Research 

conducted on the environmental impact of a submarine cable used to transmit 

hydrophonic data to shore from an acoustic hydrophone array in Monterey 

Bay, California reported no apparent effect on infaunal abundance (Kogan et 

al., 2003). Where the cable had become exposed, colonisation had occurred by 

encrusting species such as anemones, echinoderms and sponges (Figure 5.1), 

with fish congregating near the cable in places. 

At present, no specific regulations or standards exist for submarine cable 

coatings with respect to environment impacts arising from their constituents 

108


(UKCPC and Nexans Norway A/S, pers. comm.). However, cable manufacturers 

may be certified to ISO 14001 ‘Environmental System Management Certification’ 

and as part of this certification, manufacturers are required to demonstrate 

effective ways to minimise environmental risks. 

Figure 5.1: Submarine cable 3.2 cm wide colonised by Metridium 

farcimen anemones 

(Kogan et al., 2003) 

5.2.4 MITIGATION MEASURES 

Where cable installation activities are proposed within sensitive locations and 

the significance of the impact is considered to be high it may be possible to 

mitigate the effect by altering the cable route or micro-siting of the cables 

to avoid localised areas. Micro-siting of the cables was a measure that was 

recommended in the ES for Thanet offshore wind farm in order to avoid dense 

aggregations of the reef-building worm Sabellaria spinulosa (Thanet Offshore 

Wind, 2005). 

Baseline information on the distribution of sensitive habitats and species within 

the construction area can be effectively used to plan the positioning of anchor 

arrays. In this way, exclusion zones for anchoring can be established if necessary. 

Disturbance due to anchors can be further reduced by using tenders to lift the 

anchors rather than dragging them across the seabed. 

Where there are species that are particularly sensitive to increases in suspended 

sediment occurring close to positions of cable burial, it is recommended that 

109



the technique that would result in the lowest release of sediment is utilised 

whenever this is possible. 

It is important when installing cables through hard substrate that does not 

naturally infill following cable burial, such as bedrock, gravel, hard clays, that, 

when possible, techniques are used to back fill the material to ensure that a berm 

is not left. Backfilling the trench will ensure that species recovery occurs quicker 

and that obstacles are not left on the seabed. Utilising installation devices that 

possess depressors, designed to infill plough furrows, can effectively mitigate 

the impact and reduce the need for manual backfilling to occur. 

5.3 Intertidal Habitats 

5.3.1 INTRODUCTION 

The main intertidal and shoreline habitats and communities likely to be 

encountered during export cable installation include: 

● 

● 

● 

● 

● 

● 

● 

110 

cliffs; 

estuaries; 

saltmarsh; 

bedrock and boulders; 

gravel and shingle shores; 

sand and mudflats; 

seagrass beds, and; 

● biogenic (living) reefs such as mussel beds ( Mytilus edulis) and the reef 

building worm, Sabellaria alveolata reefs. 

Estuaries, saltmarshes, sand and mudflats have a high ecological value, often 

being important as feeding, roosting and nesting areas for waders and wildfowl. 

Chalk platforms and boulder shores are examples of important geological 

features and typically support a wide variety of algae and marine invertebrates, 

which are distributed in distinct zones related to tolerance to exposure and 

desiccation. Seagrass beds and biogenic reefs provide a habitat for a wide range 

of associated species and can increase habitat heterogeneity and biodiversity 

in otherwise impoverished areas. Many of these habitats will have designated 

status. 

Potentially significant effects in the intertidal zone include: 

● 

● 

● 

Seabed disturbance; 

Sediment mobilisation (including potential release of contaminants); and 

Settlement of material.


Seabed disturbance 


Construction activities leading to disturbance will include provision of access 

for equipment and any specific preparations that may be necessary, such as 

the placement and excavation of anchors to assist the cable installation barge; 

construction of jointing chambers, removal of structures such as concrete facings 

on sea walls, breach of sea defence structures and excavation of trenches to the 

jointing chamber. 

Disruption to intertidal habitats will occur within the construction corridor. The 

magnitude of direct disturbance is likely to be similar for all installation techniques 

(i.e. width of trench created) but will vary with width of the shore (see Section 

5.2.2). The significance of the impact will largely depend on the environmental 

sensitivity of the area affected and is based on the following parameters: 

● 

● 

● 

Habitat type and overall distribution within the localised area and wider 

environment; 

Recoverability of habitat and species; 

Importance of the habitat/species (i.e. protected status); 

● Use of the area for feeding and/or roosting birds (see also Section 5.7); and 

● 

Use of the area for fish spawning, nursery and/or feeding grounds (see also 

Section 5.4). 

Intertidal habitats that are more sensitive to the impacts of cable burial are 

generally those that have established in more sheltered conditions, where 

natural perturbations are lower and less frequent. Such habitats include 

saltmarsh, mudflats, muddy gravel, bedrock, biogenic reef and eel grass beds. 

These habitats, in general take longer to recover from disturbance compared to 

those which are more dynamic and/or frequently disturbed such as sand flats, 

shingle beaches and mixed sediment habitats. Such dynamic habitats are likely 

to support a different assemblage of species which are better adapted to and 

more tolerant of frequent, short term disturbance, such as occurs naturally and 

is associated with cable installation. 

The sensitivity of the intertidal environment can also display distinct temporal 

patterns, associated with seasons. Examples include use of the site by wading 

waterbirds and wildfowl during the over-wintering period (see Section 5.7), 

or being of seasonal importance to important life stages of fish and shellfish 

species (see Section 5.4). 

Sediment mobilisation 

In addition to direct disturbance, there are also issues relating to sediment 

mobilisation across the intertidal area. Cable installation within the intertidal 

111



zone is very likely to be undertaken during periods of low tide, and as such the 

potential for resuspension of material is reduced. Some of the disturbed material 

will, however enter into suspension during the flood tide but the extent of this 

will depend on the sediment type and cohesiveness. Resuspension of sediment 

is not likely to be of concern where cabling occurs within cohesive or coarse 

sediments, but can be significant when cabling is undertaken in non-cohesive 

fine sediments or chalks. Potential effects of suspended sediment are detailed 

in Section 5.2.2. 

Within fine sediments there are also issues to consider in relation to potential 

contaminant release. Contaminants, such as oils and heavy metals, generally 

attach to fine sediments but certain chemicals can persist in coarser sediments. 

Disturbance of sediment can release associated contaminants into the water 

column. If contaminants reach a certain level they can cause effects on certain 

species or can bioaccumulate through the food chain. However, the effects of 

contaminant release on the environment tend to be localised and would only be of 

concern near industrialised areas. Investigation of potentially contaminated sites 

that may lie within the cable route would be considered during the screening and 

scoping phase. If nearby sites are identified where there is evidence of historic 

contamination, sediment sampling will be necessary in order to determine the 

level of concentration within the sediment. Predictions can then be made as to 

whether the release of contaminants could have an adverse effect. 

Settlement of material 

Settlement of suspended material has the potential for smothering to occur 

that could cause the burial of important or sensitive species and habitats (see 

Section 5.2.2). Given that the installation generally occurs during low tide, it is 

only likely to be the fine sediments that have been disturbed, which may become 

suspended in the water column during the flood tide. 


Access to a site requires careful planning to avoid any sensitive features, which 

can be marked to ensure effective avoidance by construction plant and staff. 

Such features include rare or notable plants for example on shingle banks or 

saltmarsh, the presence of biogenic reef areas and the presence of important 

sessile (non-mobile) communities or species. Vegetated shingle areas should be 

avoided, or a process of translocation agreed. 

Horizontal directional drilling (within Section 3.8.6) is an appropriate form of 

mitigation to avoid damage, particularly in the intertidal and landfall areas 

where habitats may be more sensitive (e.g. chalk cliffs, saltmarsh, etc.). This 

methodology has been proposed for the Thanet offshore wind farm due to the 

inter alia geological features of the area and the presence of saltmarsh habitat 

(Thanet Offshore Wind, 2005). 

112


In order to promote recovery within the intertidal zone, material displaced as a 

result of cable burial activities should be back filled. This reduces the potential 

for remobilisation of sediments and enables recovery of benthic organisms to 

occur within a much quicker timescale. Where sensitive habitats (e.g. vegetated 

shingle, saltmarsh, etc.) are present along a cable route it may be necessary to 

remove vegetation prior to installation and replant/enhance following installation. 

Stabilisation techniques may also be necessary in certain conditions. 

Guidance is available relating to translocation and enhancement for saltmarsh 

habitat in the Environment Agency/Defra publication ‘The Saltmarsh Management 

Manual’ (Environment Agency, 2005) and the Chartered Institute of Water and 

Environmental Management (CIWEM)/Royal Society for the Protection of Birds 

(RSPB) document ‘The saltmarsh creation handbook: a project managers guide 

to the creation of saltmarsh and intertidal mudflat’ (RSPB, 2005a). 

5.4 Natural Fish Resource 

The installation and burial of export and inter-array cables has the potential to 

impact upon the natural fish resource (finfish and shellfish) in a number of ways 

including: 

● 

● 

● 

● 

Habitat disturbance; 

Noise and vibration; 

Smothering and contamination; and 

Electro magnetic field generation. 


Habitat disturbance 

The following fisheries habitat types have the potential to be affected by the 

cable installation process: 

● 

● 

● 

● 

● 

● 

Spawning grounds; 

Nursery grounds; 

Feeding grounds; 

Over-wintering areas for crustaceans; 

Migration routes; and 

Shellfish beds. 

Disturbance caused by the presence of installation vessels and equipment (and 

associated noise) will displace fish within the water column from the vicinity of 

113



operations. This is seen as a localised and temporary displacement of fish, which 

in isolation is generally not a significant impact on natural fish resources. 

Most species of marine fish spawn in the water column and so changes to 

the seabed through the placement of a cable do not have severe long-term 

implications. However, disruption to the spawning of species that do utilise 

the seabed, such as Atlantic herring (Clupea harengus), sandeels (Ammodytes 

tobianus) and dogfish, should be minimised through alternative routing or 

timing to avoid spawning areas or periods respectively. 

In general, CEFAS recommend that construction on or in the seabed should be 

carried out outside of the spawning season for substrate spawners e.g. February 

to April for spring spawning herring. As this is an important operating window, 

it may be prudent to determine the extent of spawning areas within the cablelaying 

corridor. In the absence of data regarding the importance of sites for 

spawning, additional studies may be required to determine whether mature fish 

in spawning condition are present in the area during the spawning season and/ 

or whether eggs and larval stages are present (see CEFAS, 2004). 

Nursery grounds (areas favoured by juvenile fish) are also important habitats, 

although in many locations such habitats may be widespread. If the cable pathway 

crosses through an important nursery ground, then the relative importance of 

the site to that region should be assessed. 

As most fish species are relatively opportunistic predators, particular feeding 

areas are not well defined. However, some species of fish may congregate in 

certain areas at particular times of the year to feed on particular prey species. 

The routing and laying of the cable should therefore minimise disruption to such 

sites wherever possible. 

Habitat disturbance can be a more significant issue to benthic (associated 

with the seabed) mobile fish resources such as many shellfish species. This is 

particularly relevant for areas which are important for certain shellfish life stages 

where there is reduced mobility, namely crustacean over-wintering areas and 

settlement areas for juvenile shellfish. Settlement areas are discussed further in 

‘Smothering’ section below. 

The true extent of seasonal migration by many fish and shellfish species is 

becoming better understood. Again the severity of impact from cable installation 

activities is likely to increase for species that migrate across the seabed, namely 

crustacean species such as lobster (Hommarus gammarus), spider crab (Hyas 

araneus) and edible crab (Cancer pagarus). Routing and timing of cable-laying 

operations to avoid disruption to this seasonal activity may be required. For 

example, for the Norfolk (Cromer) Round 1 Offshore Wind Farm, mitigation 

cited in the Environmental Statement recommended that the export cable, which 

covered an inshore area believed to be important for “pairing” (or mating) of 

edible crab, should be installed outside this period (July-September) (Norfolk 

114


Offshore Wind, 2002). The presence of this local crab fishery, which is considered 

to be of local and national importance, led to a FEPA consent condition requiring 

a crab surveillance programme to be formulated (Cefas, pers. comm.). 

Molluscan shellfish species, such as king scallop (Pecten maximus), mussel 

(Mytilus edulis), native oyster (Ostrea edulis) and cockle (Cerastoderma edule), do 

exhibit avoidance behaviour over very short distances, but are less mobile than 

crustacean species and likely to be directly impacted by trenching operations. 

For most shellfish beds any resulting mortality along the width of the cable route 

is generally not a significant loss in relation to the remaining population. For 

small, isolated beds, however, the physical damage and disturbance from cablelaying 

operations can be significant. 

Mussels settling on the seabed attach to surrounding hard surfaces, including 

other adjacent mussels. This behaviour can alter the seabed substrate with the 

formation of biogenic reefs. These reefs can extend over significant areas of 

seabed creating a rich habitat for other species as well as suitable habitat for 

further mussel settlement. Such reefs are widespread, but generally found in 

large shallow inlets and bays in estuarine areas (Holt et al., 1998; BMT Cordah, 

2003). Ploughing through these reefs can in some instances destabilise the reef 

by leaving exposed edges prone to damage by wave and current action. While 

biogenic reefs formed by Mytilus are found to be more tolerant of disturbance 

than other types of biogenic reef such as Sabellaria reefs (Holt et al., 1998) (see 

Section 5.3), these species-rich habitats should be avoided where possible. 

Noise & vibration 

The potential impact of noise and vibration on the natural fish resource within 

the affected area will be largely dependant upon the ‘hearing’ sensitivity of the 

fish species concerned. In this context three main types of fish are recognised: 

● 

● 

● 

Hearing specialists: These species, including herring and sprat, ‘hear’ sound 

through the acoustico-lateralis system; a collective term for the inner ear and 

lateral line. Sound vibrations are also detected by a gas-filled swim bladder 

which is connected to the inner ear via a gas duct; 

Hearing specialists with mid range sensitivity: These species, including cod, 

mackerel and salmon, are hearing specialists but are deemed less sensitive 

to noise, due largely to the lack of a gas duct between the inner ear and the 

swim bladder; and 

Non-hearing specialists: Typical species include flatfish such as dabs, plaice 

and sole and elasmobranchs such as dogfish. These species are non hearing 

specialists and do not possess a swim bladder. 

115



Noise associated with cable laying will therefore impact upon hearing specialists 

to a greater extent. The effect of underwater noise on fish can be categorised as 

(Nedwell et al., 2003): 

● Primary effects: These include immediate or delayed fatal injury of animals 

near to powerful sources, such as the blast from underwater explosives; 

● Secondary effects: These include injuries and deafness which may have long 

term implications for survival; and 

● 

116 

Tertiary effects: These are most likely to be associated with cable laying and 

include avoidance of the area which could have significant effects in the 

vicinity of breeding grounds, migratory routes or schooling areas. 

Nedwell et al. (2003, 2005) proposed a measure of sound that takes into account 

the differences between species in terms of hearing ability. This measurement 

is referred to as dB ht (Species). The derivation of dB ht (species) involves the 

measurement of sound passing through a filter that mimics the hearing ability 

of individual species and provides a species specific measurement of the 

likely level of perception of sound by the species. For a full description of the 

dB ht (Species) measurement and the uncertainties involved in its application, 

the reader is directed to Nedwell et al. (2003, 2005). While the dB ht (Species) 

approach is still being validated and reviewed, available information suggests 

that species of fish and marine mammals (see Section 5.6.1) will show a strong 

avoidance reaction to sound levels of 90 dB ht and above. Strong avoidance by 

most individuals is likely to occur at 100 dB ht with a mild avoidance reaction 

occurring in a minority of individuals at levels above 75 dB ht . The effect of such 

impacts will be dependant upon a combination of factors including the type and 

magnitude of the noise along with the proximity of an organism to the source 

of the noise. 

In the context of offshore wind farm construction, much of the work relating to 

the impact of noise upon species has focussed on the effect of pile driving, as 

this is by far the ‘worst case scenario’ in terms of the production of high intensity 

impulsive sound. Sound levels associated with cable installation have received 

considerably less attention and very little monitoring data is available. 

Nedwell et al. (2003) have reported that cable trenching in sandy gravel at North 

Hoyle offshore wind farm produced noise at a source level of 178 dB re 1μPa @ 

1m. However, when illustrating the dB ht levels of the noise as a function of range, 

the following figure was produced. 

Although based on some uncertainty, due to the high levels of variability in the 

noise produced, Figure 5.2 clearly shows that, for each species the dB ht level is 

below 90. In this situation, significant avoidance reactions amongst fish would 

not be expected to occur.


Figure 5.2: Cable burial noise (North Hoyle Offshore Wind Farm) 

Source: After Nedwell et al., (2003) 

The example in Nedwell et al. (2003) only covers one installation method in 

one type of substrate. It is possible that certain burial tools in certain seabed 

sediments may produce noise at a higher level than recorded at North Hoyle. 

Further information is required on the noise levels associated with other forms of 

cable installation before any clear guidance on the expected levels of associated 

disturbance to fish and/or mammals can be made. However, the early indications 

are that there is no significant impact from cable burial noise on fish species. In 

addition, no harmful events have been reported from the well established subsea 

telecommunications industry. For a more detailed assessment of the effects of 

man made noise on fish see Nedwell et al. (2003). For further information on 

noise effects on marine mammals see Section 5.6.1. 

Turbidity 

A reduction in light levels within the water column can create a number of 

adverse effects on fish and shellfish resources. Long term increases in turbidity 

will reduce the extent of the photic zone (the depth to which light can penetrate 

the water column) resulting in changes to flora and fauna. Cable laying operations 

result in temporary increases in suspended sediment which, while generally not 

sufficient to alter biotopes, will impact upon sensitive species or those reliant on 

certain levels of visibility. 

Decreased visibility through increased concentrations of suspended sediments 

can affect predatory fish such as mackerel (Scomber scombrus) and turbot (Psetta 

maxima), which rely on vision to detect and locate prey, leading to decreased 

117



feeding efficiency. In addition many fish species such as herring rely on light 

levels to aid migration and shoaling behaviour. Low light levels caused by high 

levels of suspended sediment could impair the ability of species to shoal as part 

of migrations to spawning or feeding grounds. Studies described in Section 4.4 

indicate short term and localised increases in turbidity over a number of tidal 

cycles (for those cable installation methods investigated). 

Smothering 

The blanketing or smothering of benthic animals and plants, may cause stress, 

reduced rates of growth or reproduction and in the worse cases the effects may 

be fatal (Bray, Bates & Land 1997). The impact of smothering on fish and shellfish 

will be a function of the settling behaviour of sediment resulting from increased 

suspended sediment concentrations relative to background levels, the sensitivity 

of certain species and/or lifestages to those increases and their ability to move 

to other areas. The significance of this impact is dependent on many variables 

including hydrography, seasonality, sediment type, species and the technique 

used to bury the cable (see Sections 4.2 and 4.3 for more details). 

The main impact on fish is the irritation and clogging of gills. Juveniles are more 

susceptible to this as adult fish would normally be able to detect significantly 

elevated levels of suspended sediment and move away from the affected area 

(ABP Research, 1997). 

Smothering can result in significant mortalities on shellfish beds as they are 

less mobile than fish species, with many having lifestages that are sensitive to 

variations in sediment particle size within the water. Respiratory and feeding 

apparatus may be clogged by the settlement of significant amounts of sediment 

that is mobilised by cable-laying operations. Filter feeders such as mussel, oyster 

and scallop are therefore among the most vulnerable to smothering effects. 

Shellfish are particularly susceptible during spring when spatfall occurs (Posford 

Duvivier & Hill, 2001). If sensitive spawning or shellfish beds cannot be avoided 

entirely, seasonal avoidance may be required. 

Water quality 

Cable installation requires extensive use of shipboard and subsea equipment 

which in many cases is hydraulically driven. There is a consequent risk of spillage 

of hydraulic fluid from vessels. The spillage is, however, limited by the system 

design. Perhaps the greatest risk is a major leak from a burial ROV system when 

operating on the seabed. Such systems have typical reservoir capacity of 60-100 

litres. Spillage risks can be minimised by good practice: 

● 

● 

118 

Deck mounted hydraulic equipment should be fitted with saveall (cofferdam) 

surrounds to catch leakage and prevent discharge over the vessel side; 

Spare hydraulic oil should be securely stowed, preferably below decks;

● 

● 

● 

● 


Oil levels should be monitored regularly to safeguard against undetected 

leakage; 

Hydraulic systems should be designed to limit discharge in the event of a 

system failure with use of no return valves where possible; 

ROV hydraulic systems should not be all fed from a single common reservoir, 

but have a number of separate reservoirs to limit any potential impact from 

a system failure; and 

Appropriate spillage control procedures and equipment should be available 

on board the operations vessels. 

Other potential sources of pollution include lubricants used during horizontal 

directional drilling methods of cable installation. These are generally inert 

biodegradable substances such as bentonite which will disperse rapidly. 

Some localised and short term aesthetic impacts on the sea surface may be 

experienced. 

Water quality (and pollution prevention) will also be an important consideration 

where cable installation takes place within or near a designated Shellfish Water. 

The EU Shellfish Water Directive (adopted in 1979) outlines the requirements for 

the quality of designated waters which support shellfish (defined as bivalve and 

gastropod molluscs) and aims to protect these shellfish populations from the 

harmful consequences resulting from the discharge of polluting substances into 

the sea. This Directive has been transcribed into UK legislation under the Surface 

Waters (Shellfish) (Classification) Regulations 1997 and The Surface Waters 

(Shellfish) Directions 1997. Cable laying in these areas should be avoided; where 

avoidance is not possible, close liaison with fishery operators and management 

authorities, including the Environment Agency is recommended. 

Electro Magnetic Field (EMF) 

The transport of electricity through an export and inter-array power cable has the 

potential to emit a localised electromagnetic field (EMF) which could potentially 

affect the sensory mechanisms of some species of marine fauna. The degree 

of impact and the subsequent effect on marine communities was investigated 

by The Centre for Marine and Coastal studies and Cranfield University in 2003 

and 2005, funded through Collaborative Offshore Wind Research into the 

Environment (COWRIE). The COWRIE investigations found that the EMF emitted 

by industry standard AC offshore cables had a magnetic field component and 

an induced electric field component (COWRIE, 2003). These EMF components 

were both within the range of detection by EM-sensitive aquatic species, such 

as sharks and rays (Elasmobranchii). 

It is generally acknowledged that elasombranchii will encounter multiple EMF 

components within the wind farm from the inter-turbine array and as a linear 

field from the export cables. From previous research it has been shown that 

iE-Fields can affect the behaviour of elasmobranchii when they reach a critical 

119



level. The main potential impact is the disruption of the sensory cues for feeding 

in benthic dwellers in and around the wind farm area. This is general for all 

species of benthic dwelling elasmobranchii that feed diurnally and nocturnally. 

EMF could have two possible effects on the behaviour of elasmobranchii. Firstly, 

resident elasmobranchii could be deterred from feeding in and around the area 

within the wind farm footprint and where cables are buried. The second impact 

could be one of attraction of elasmobranchii to the footprint of the wind farm 

potentially causing an unnatural clustering effect in the area. 

There are, therefore, potentially significant effects on fisheries interests but to 

date there has been no evidence to indicate the likelihood and/or magnitude of 

these. The research undertaken by COWRIE, under laboratory conditions only has 

been insufficient to determine the precise extent of detection for many marine 

species; the behavioural response within the zone of detection and therefore the 

likely impacts on fisheries resources due to EMF from sub-sea cables (COWRIE 

2005). 

For the Thanet Offshore Wind Farm site, English Nature stated that (given current 

levels of ecological understanding) there will not be a significant impact to the 

populations of elasmobranchii that are resident within the wind farm footprint 

and cable export route but English Nature’s advice is provided in the light of 

current information that is available (English Nature, 2006). 

Additional research work on electromagnetic field effects is due to be carried out 

under the auspices of COWRIE, which will entail “in the field” research. It is likely 

therefore that developers will be asked to commit to take on board the most up 

to date information on electromagnetic field effects, thereby making necessary 

and reasonable adaptations during the construction, operation and monitoring 

of wind farm developments. 


In terms of the natural fish resource, general mitigation measures should seek 

to do the following: 

● 

● 

● 

120 

Avoid spawning and nursery habitats. Such information should be available 

from published sources or previous surveys. If this is not available then a 

series of dedicated surveys should be commissioned as part of the wider EIA, 

in discussions with Cefas; 

Avoid sensitive spawning times for substrate spawning species where 

possible (exact periods will be site specific and require evidence); 

Impacts from noise may be mitigated by timing construction to avoid sensitive 

feeding, spawning and nursery area/times of the year;

● 

● 


Use techniques which minimise re-suspension of sediment in areas where 

biotopes sensitive to smothering are present and where sediment is found to 

have elevated levels of pollutants; and 

To minimise pollution risk the laying of the cable should be undertaken 

in accordance with an Environmental Management Plan and general 

environmental good practice on site (e.g. CIRIA Marine Construction Site 

Guide). 

5.5 Commercial Fisheries 

Impacts to commercial fisheries may result from effects to the targeted fisheries 

resource or due to alterations to fishing practices as a consequence of cablelaying 

operations. This section details the impacts of cables and cable installation 

relating to commercial fisheries over and above those identified in the natural 

fisheries section (Section 5.4). 

Commercial fishing activity can be broadly divided into two approaches: mobile 

gear (where the nets or lines are towed by vessels) and static gear (where nets, 

lines or pots are left in the environment for a period of time) 1 (Table 5.1). The 

vessels used for these various methods may have some degree of specialism. 

Table 5.1: Commercial fishing gear: mobile gear and static gear 

Mobile Gear Vessels Static Gear Vessels 

Beam trawler Drift netter 

Seine netter Gill netter 

Trawlers (e.g. demersal, freezer, pair) Long liner 

Dredgers (e.g. scallops, cockles) Potter/whelker 

Rod and line 

Both mobile and static gear vessels have the potential to be impacted by cable 

installation. The main effects of cable laying on commercial fishing activity, of 

potential significance are: 

● 

● 

● 

Restricted access to fishing grounds; 

Temporary fish stock displacement; and 

Snagging of fishing gear. 

Each type of impact can lead to reduced returns and/or increased costs that 

result in reduced profitability for the sector. However, the actual significance of 

the effect would be highly site specific and dependant upon the duration of the 

installation process. 

1 

A variation on static netting gear can involve nets being set, but not anchored, enabling them to drift in the current: 

drift netting. 

121



The greatest risk associated with fishing-cable interactions is to trawlers that 

may ‘snag’ a cable, which can pose a significant danger to the vessel and its 

crew, if not properly managed. Guidance for the fishing industry regarding 

submarine cables is presented in “Fishing and Submarine Cables Working 

Together” produced by the International Cable Protection Committee (ICPC) 

(Drew & Hopper, 1996) and free charts which ICPC issue to fishermen to plot 

the routes of submarine cables. This information is intended to help fishermen 

avoid snagging submarine cables and to provide information about what to do 

if a cable becomes caught in fishing gear. 

5.5.1 POTENTIAL EFFECTS 

Access to fishing grounds 

Fishing vessels are prevented from fishing within the immediate area during 

cable laying and burial operations. As a result there may be short term restrictions 

to fishing grounds. In some areas, fishing vessels affected will be temporarily 

displaced to adjacent fishing grounds, which can lead to an increased risk of 

gear conflict and a temporary reduction in catches where fishermen are forced 

to fish unfamiliar or less favourable grounds. 

During cable installation operations a safety zone surrounding work boats 

should be established; this can result in increased steaming times to fishing 

grounds and hence increased fuel costs and reduced fishing time or a longer 

working day. Again this is a short-term impact that may be difficult to quantify 

as fishing vessels will often target several fishing grounds and so should be able 

to fish alternative grounds to avoid short-term restrictions. The landing of the 

cable in constricted areas (e.g. narrow estuaries) can interfere more significantly 

with fishing vessel movements as vessels may be restricted in departing from 

or returning to port/berths. 

As restrictions are likely to be short term, impacts are generally thought to be 

minimal. For example, the two export cables from the North Hoyle wind farm 

to the landing beach, a distance of 7.5 km, took approximately 4 to 5 days each 

to lay and bury (Bomel Ltd., pers. comm.). Furthermore, one of the landing 

routes under consideration for the North Hoyle project up the Rhyl estuary was 

abandoned in favour of alternative route, partly because of the disruption this 

would have caused to the local fishing fleet (Bomel Ltd., pers. comm.). Therefore, 

the likely impact on access to commercial fishing grounds is likely to be small 

given the wider impacts of the wind farm. 

Fish stock displacement 

During construction there will be a short term movement of fish away from the 

cable laying area due to inter alia noise, seabed habitat alteration and turbidity 

(see Section 5.4). 

122


There are two mechanisms by which fish and shellfish may be displaced from 

the installation area. One is direct avoidance of the burial site in response to 

noise, the turbidity plume or both. This will be a local effect of short duration that 

will cease once the cable has been installed (see Sections 4.3 and 4.4). The other 

is in response to sea bed habitat alteration that may well downgrade the cable 

route as a feeding area. Due to the short duration of construction activity and the 

small width of habitat affected by cable laying, the likely impact on commercial 

fishing is expected to be small. 

The temporary displacement of fish stocks through disturbance or habitat 

alteration is predicted in the Environmental Statements for the Horns Rev and 

the Nysted wind farms, Denmark. The short term nature of this impact has been 

corroborated by subsequent monitoring studies. The most recent monitoring 

report available from 2004 concludes for Nysted that: 

“the direct and indirect impacts of the earthwork [conducted from September 

2002 to February 2003] on eelgrass, macroalgae and invertebrates were 

limited in space and time and a full recovery of the populations close to 

the cable trench is expected in the near future.” (Elsam Engineering & 

ENERGI E2, 2005). 

For Horns Rev, which forms part of Denmark’s important commercial sandeel 

fishery, the 2004 monitoring report found that “There is no indication that 

the construction of the wind farm has had a marked effect on the sediment 

composition in the wind farm area. There was no indication of an increase in 

the content of silt/clay and very fine sand in the impact area from 2002 to 2004. 

Sandeels are very sensitive to changes in the content of these sediment sizes and 

will completely abandon the area if the weight fraction of the silt/clay content 

rises above 6%.” Sediment movement resulting from cable laying was found to 

be insignificant in the relatively dynamic environment of the wind farm. 

There is concern amongst the fishing industry of the potential impacts on fish 

stocks of EMF emitted by operating power cables (see Section 5.4). This is 

particularly important to small vessel operators as these vessels have a limited 

operating area from their home port and therefore may be unable to still target 

highly displaced resources. Monitoring of the earliest-established offshore 

wind farms, Horns Rev and Nysted, have to date found no significant impact of 

either the export cable or the inter-array cables on fish stock displacement. The 

creation of significant areas of hard substrate through placement of the turbines 

and associated rock armour has, however, led to a changed (moderately 

increased) epibenthic productivity (macroalgae and benthic invertebrates) that 

has attracted commercial fish species within the turbine array. There has been 

no reported reaction of these fish species to the presence of cables with localised 

emissions of EMF. 

The permanent alteration of the seabed will be most pronounced with the use 

of rock dumping to protect cables when burial is not feasible and the installation 

of concrete mattresses at cable crossings. While this is seen as predominantly 

123



a negative impact for commercial fisheries, the introduction of hard substrate 

creates habitat diversity, which can benefit some commercial resources e.g. 

crustacean fisheries. 

Mobilised sediment may settle on fishing gear (e.g. monofilament gill nets) 

and reduce their efficiency. The significance of this depends on background 

turbidity levels and given the dynamic hydrographic conditions of much of the 

UK coastline this is likely to be insignificant. 

Snagging 

The extent of the physical disturbance to the seabed along the cable route will 

depend on the prevailing geological conditions, which determine the cable 

installation techniques that can be used. For soft sediment, natural infilling 

or assisted infilling by the cable burial tool should ensure that no debris to 

potentially snag a fishing net is left on the seabed. However in rocky substrata 

or in clay, chunks of debris could potentially cause fishing nets to snag. 

Snagging can also occur where stretches of cable are exposed for example 

over rocky substrata, spanning sand waves or at any location where cable is 

surface laid prior to any post lay burial operation. However, there should not be 

a problem for the latter point, as the fishermen should be respecting exclusions 

zones until all cable burial operations are complete. Shifting sediment can also 

expose previously buried sections of cable. 

Seabed hazards resulting from the trenching of the cable route, including any 

exposed sections of the cable itself, can interfere with benthic fishing gear, 

particularly beam trawl, otter trawl and drift netting. Static gear also has the 

potential to become snagged on subsea cables. The small 1m long anchors used 

by inshore vessels to secure static gear are found to penetrate sand to a depth of 

around 0.2m (less in coarser sediments). These may drag in strong currents and 

become snagged on cable. The risk lies in attempting to recover the gear, which 

is often through the use of grappling hooks. The cable route assessment should 

however take on board all of these potential hazards and the design depth of 

burial for the cables should provide suitable mitigation against the measured 

risks. 

Danger arises where vessels, unknowingly snagged on a cable attempt to heave 

the gear free from the seabed obstruction. This can lift the cable clear of the 

seabed, exposing more cable that causes a significant downward pull from the 

weight of cable. 

The recovery of snagged gear is not recommended and fishing operations will 

be compensated by cable operators if provided with evidence of such instances. 

Such claims are only compensated providing that the fishermen have not 

operated within restricted fishing or anchorage zones. 

124



The extent and risk ‘associated’ with interactions between cables and fishing 

vessels should be avoided or minimized through appropriate: 

● 

● 

● 

Routing of cables and scheduling of cable laying operations; 

Cable laying technique to ensure sufficient cable burial and/or protection; 

Communication of cable laying activities and cable positions (via Notices to 

Mariners, Fishing News, Kingfisher Bulletins and Admiralty Charts); and 

● Monitoring of the cable in situ to identify and address areas of impact and 

risk to fisheries. 

The choice of cable route and cable laying techniques would be determined 

following an assessment of existing commercial fishing activities in the area and 

the sensitivities of the resources upon which they depend. 

The use of best engineering practices would be employed e.g. ensuring 100% 

of the cable route had adequate protection with no exposed sections of cable 

wherever possible. 

Using suitable local fishing vessels as guard vessels for cable laying operations 

provides useful alternative income to the fishing industry. 

Monitoring of the cable route post installation (e.g. side scan sonar) and 

regular communication with fishermen should help minimise impacts and risk 

from construction operations. Communication with fishermen will be greatly 

facilitated with the use of a suitable fisheries liaison officer. 

5.6 Marine Mammals 

Marine mammals 2 have a wide ranging distribution and, as such, there are 

no marine mammals considered to be exclusively British. The cetaceans most 

commonly encountered and described as part of offshore wind farm projects are 

the harbour porpoise Phocoena phocoena and the bottlenose dolphin Tursiops 

truncatus. Harbour porpoise are by far the most abundant 3 (Hammond et al. 

2002). 

Two species of seal are resident in UK waters; the common seal Phoca vitulina 

and the grey seal Halichoerus grypus, with the grey seal being more numerous 

(English Nature, 2004). 

The range of potential impacts upon marine mammals during cable installation 

(and, to a much greater extent, offshore wind farm construction) are of 

2 In the context of this Technical Report, ‘marine mammals’ is the collective term for seals (pinipeds) and whales, 

dolphins and porpoise (cetaceans). 

3 For details of other cetacean species occurring in northwest European waters see http://www.jncc.gov.uk/ 

page-1554 

125



particular significance, given that each of these animals is afforded a high level 

of individual protection under a suite of national and international legislation 

and signatory agreements 3 (Defra, 2005). As such, potential adverse impacts 

upon a single individual of a species of marine mammal must be considered as 

being of significance and, therefore, there is a requirement to apply practicable 

and financially feasible mitigation measures in order to be compliant with the 

legislation (in particular, the Habitats Directive). 


Cable laying operations have the potential to impact upon marine mammals 

through: 

● 

● 

● 

● 

126 

Collision with the vessel or support vessels; 

Noise and visual disturbance from the vessel and cable burial system; 

4 Contact with any fuels and chemicals that may be accidentally released 

during the operation; and 

Interactions or entanglement with the cable or other lines between the vessel 

and the installation tool. 

Collision risk 

While there are no accurate records of the number of incidents of accidental 

collisions between marine mammals and shipping in UK waters (e.g. Hammond 

et al., 2003), it is considered that a direct relationship exists between shipping 

intensity, vessel speed and the number and severity of collisions, certainly in the 

case of whales (Laist et al., 2001). Certain assumptions can be made about the 

risk of collision with marine mammals (Sakhalin Energy Investment Company 

Limited, 2005; Laist et al., 2001): 

● 

● 

● 

● 

● 

All types and sizes of vessels can hit marine mammals; 

Vessels over 80m in length cause most severe or lethal injuries; 

Serious injuries to mammals rarely occur if struck by vessels travelling at 

speeds of less than 10 knots; 

Mammals struck by vessels are usually not seen prior to impact, or are seen 

too late to avoid impact; and 

The risk of collision increases in poor visibility. 

Collisions between marine mammals and the cable burial vessel are considered 

to be unlikely. The cable burial vessel whilst working in the shallow water 

locations, is likely to only make very slow progress while laying and burying 

cable, with a burial rate of 1000m of installed cable per hour and therefore would 

4 Such accidental releases are likely to be similar in extent, duration and significance as for other parameters and is 

not discussed in this section.


not be travelling at over 10 knots during installation operations, thus reducing 

the risk of fatal injury, should a collision occur. 

Visual disturbance 

Marine mammals may be disturbed by the presence of vessels and human 

activities, particularly in sensitive locations such as in close proximity to seal 

haul out sites and marine mammal foraging areas. Such disturbance, during 

sensitive periods (such as the breeding and pupping season of seals), may 

lead to significant impacts such as the abandonment of young and reduced 

reproductive success (Brown and Prior, 1997). 

Underwater noise 

There is increasing concern over the impacts of underwater noise as a result 

of anthropogenic activities upon marine life in general. This issue is of greater 

relevance to marine mammals, given both their physiological capacity for 

detecting and responding to sound, and the high levels of protection that they 

are afforded. 

The sources and intensities of sound associated with offshore wind farm 

construction and the related impact on marine life has been investigated by 

Nedwell et al. (2003) and Nedwell & Howell (2004). Further useful information is 

provided in Jansy et al. (2005) and Madsen et al. (2006). The impact of noise on 

marine mammals can be divided into three levels: 

● 

● 

● 

Those that cause fatal injury; 

Those that cause non-fatal injury such as deafness and other auditory damage 

such as temporary threshold shift (TTS); and 

Those that cause behavioural change (e.g. avoidance, cessation of feeding 

etc.). 

Similarly to the impacts of underwater noise on fish, available information 

suggests that species of marine mammal will show a strong avoidance reaction 

to sound levels of 90 dB ht (species) and above. It is, however, considered highly 

unlikely that cable installation would produce noise at a level that would cause 

a behavioural reaction in marine mammals. For a more detailed discussion of 

the dB ht (species) measurement, an example of where the measurement has 

been applied to cable installation and the likely levels of effect on both marine 

mammals and fish, see Section 5.4.2. 

Entanglement 

The risk of marine mammals becoming entangled in a cable under low 

tension, or in any other lines used to connect the installation tool to the vessel, 

is considered to be extremely limited. Most seals and cetaceans would be 

expected to avoid areas of human activity and significant disturbance and, as 

127



such, would be unlikely to venture close enough to the cable burial vessel to 

become entangled. A review of relevant published literature and discussions 

with installation contractors has not identified any reports of marine mammals 

of any kind becoming entangled during cable burial operations. 


Visual and other construction related disturbance, in relation to hauled out seals, 

can be effectively mitigated by avoiding cable installation operations in the 

vicinity of known haul out sites during sensitive periods, such as the breeding 

season (late June to early July for common seal and late July to early December 

for grey seals (although this varies with position around the UK). Wherever 

possible, seal haul out sites should be avoided during the planning of the cable 

route in order to completely remove the potential for disturbance to occur. 

Further study is required to assess the noise levels produced by the range 

of available cable burial devices and tools in the types of seabed sediments 

encountered in UK waters. This can be achieved through real time monitoring 

of cable installation, such as at North Hoyle, or through specific experimentation 

and computer modelling. Only once sufficient, reliable data is available can 

the disturbance caused by cable installation be understood and effectively 

mitigated. As a precautionary measure, given the conservation significance of 

the species involved, it may be necessary, as a minimum, to employ Marine 

Mammal Observers on the installation vessel and to have a protocol in place 

to delay installation activities from occurring if marine mammals are detected 

within a predetermined distance from the installation vessel. Such a need would 

be identified through consultation with the relevant nature conservation body 

and the JNCC (JNCC, 2004). 

5.7 Ornithology 

Concerns and issues relating to birds, in the context of offshore wind farms are 

primarily concentrated on the wind turbine array. The installation of both the 

export and inter-array cabling is of significantly lower concern. 

Potentially significant effects of cable installation on birds are limited. The main 

area of concern would be: 

● 

● 

128 

Disturbance of normal behaviour in the intertidal; and 

Disturbance of normal behaviour at sea. 

Other effects on birds would be limited to: 

● 

Prey availability.


Disturbance of normal behaviour in the intertidal 


In the intertidal zone, the impact of disturbance upon birds may be significant. 

The presence of construction plant and activity is likely to cause temporary 

disturbance to birds which would otherwise be foraging, loafing and roosting. 

The significance of the impact depends upon a number of factors, including the 

importance of the intertidal site to birds, the duration of intertidal works and 

the season in which the works are programmed. Such disturbance would be 

particularly significant if the intertidal area in question is designated as an SPA. 

Disturbance of normal behaviour at sea 

The installation of export cables involves activity in shallow coastal waters that 

can provide favourable foraging habitat for a wide range of seabird species, 

particularly in areas of shallow sandbanks. Noise levels and the presence of 

vessels and machinery associated with the installation process may impact 

on the use of the area by foraging birds, or those ‘rafting’ on the sea, through 

disturbance of normal behaviour. However, given the transient nature of cable 

installation, such disturbance will be highly limited, both spatially and temporally 

and would not be of major concern. 

5.7.2 OTHER EFFECTS 

Prey availability 

Impacts on the seabed, and within the water column, that cause direct removal 

or displacement (i.e. due to noise) of fish and benthos, may indirectly impact 

birds through a reduction of prey availability in favoured foraging habitat. 

Offshore, foraging seabirds target a range of different fish species, particularly 

in shallow coastal waters. If installation activity causes the movement of prey 

species out of the area, this is likely to result in a related displacement of bird 

species that would follow the prey. The impact of displacement would be limited 

in both extent and duration and not considered as being significant, unless the 

cable is to be installed in an area that supports an important prey resource that 

has a limited distribution in the wider study area and is associated with a sea 

area considered to be suitable for the designation of offshore SPAs 5 . 

Similarly, in the intertidal zone, there may be a short term decrease in prey 

abundance caused by the direct loss of flora and fauna in the footprint of the 

installation device. The significance of this impact would depend on the availability 

of similar prey within the intertidal, outside the installation footprint. 

5 http://www.jncc.gov.uk/page-1414 

129




For installation works in the intertidal zone, the approach to appropriate 

mitigation will be determined by the sensitivity of the habitat. If installation 

is to take place within an area of importance to birds, such as an SPA, it will 

be necessary to ensure that the measures proposed are sufficient to avoid an 

adverse effect on the integrity of the designated site. In such circumstances, it 

will be necessary to agree mitigation with the relevant nature conservation body 

as part of the EIA process. 

In many cases the SPA will be designated for supporting over wintering 

assemblages of waders and waterfowl. Restrictions may be placed upon works 

occurring during the over wintering period (1 st October to 15 th April in any one 

year) or other sensitive times for specific species (e.g. breeding periods for 

Annex I birds, such as little tern). 

Offshore, practical implementation of mitigation measures aimed at minimising 

impacts on birds is extremely difficult. In particularly sensitive areas, such as 

shallow sandbanks, avoiding key times of year in terms of peak numbers of 

birds feeding over known areas of the cable route would ensure that significant 

disturbance is avoided. However, such an approach is likely to be difficult to 

achieve, unless there is reliable and robust information on the locations and 

seasonality of important prey resources in the study area. Also, given the 

transitory and temporary nature of the installation, the effect upon birds, at sea 

is unlikely to be significant. 

Examples of operational restrictions that have been placed on offshore wind 

farms through the FEPA licences in order to minimise the impacts on birds, 

include: 

● 

130 

Kentish Flats 

“The Licence Holder must ensure that if cable installation occurs between 

October and April inclusive (the over-wintering season for several wader 

species) the beach installation, including trenching and cable laying, 

avoids the sensitive period 2 hours either side of high water. The Licence 

Holder should also investigate putting in place acoustic shielding around 

all construction activities on the beach and at the adjacent construction 

compound in Hampton Pier car park to further minimise any potential 

disturbance.”

● 

Barrow 


“As there are internationally important numbers of common scoter in the 

vicinity of the wind farm, the Licence Holder must ensure that works are 

undertaken in the months of March to October (inclusive) so as to minimise 

disturbance to over-wintering birds. Any specific requirement for works outside 

these times shall only take place after written approval from the Licensing 

Authority (following consultation with CEFAS and English Nature). In so far as is 

practicable, the majority of the piling or drilling works shall only be undertaken 

during the months of April to June.” 

Where technically and economically feasible, effective mitigation could be 

applied through the final choice of the cable installation technique. For example, 

the use of horizontal directional drilling (as opposed to ploughing and trenching) 

would reduce the area of intertidal impacted and involves shorter periods of 

human presence in areas sensitive to disturbance. 

5.8 Shipping and Navigation 

A maritime traffic survey, navigation assessment and a Navigation Risk 

Assessment will be required for the offshore wind farm development, in 

accordance with the requirements of the Maritime and Coastguard Agency’s 

(MCA) Marine Guidance Note MGN 275(M) 6 . Information from this work can be 

used to identify and evaluate potential impacts regarding the cable installation. 

A Guidance Document is also available on the ‘Assessment of the Impact of 

Offshore Wind Farms: Methodology for Assessing the Marine Navigational 

Safety Risk of Offshore Wind Farms’ (DTI, 2005a). The purpose of the guidance 

document is to provide a template to be used by developers in preparing their 

navigational risk assessments, and to assist Government Departments in the 

assessment of these. 

The key impact associated with cable installation activities will be: 

● 

Increased risk of collision by existing navigational users in the cable 

installation area. 

POTENTIALLY SIGNIFICANT EFFECTS 

Collision risk 

Due to the increase in the number of vessels and movement of the vessels 

involved in cable deployment and burial, there will be some temporary and 

minor disruption to navigation. There are a number of particular factors which 

6 Proposed UK Offshore Renewable Energy Installations (OREI) – Guidance on Navigational Safety Issues 

131



will affect collision risk for vessels undertaking cable installation (or maintenance) 

which can be summarised as follows: 

● 

● 

● 

132 

The navigation (i.e. direction and manoeuvrability) of cable installation 

vessels is restricted by the cable and any burial equipment operated from 

the vessel. Laying speed is slow (anywhere between 100m/hr to 1000m/hr), 

and the heading of the cable installation vessel may well be set to minimise 

environmental forces from wind and current. The cable installation vessel 

is likely to be operating subsea equipment, and may be moored by anchors 

which will be deployed some hundreds of metres from the vessel. All these 

factors may not be anticipated by passing navigational traffic. Although Notice 

to Mariners are issued via the Marine Coastguard Agency they do not always 

get received by all intended recipients. For example, during the installation 

of power cables across the Solent in the late 1990’s a power boat race came 

very close to the mooring wires of the main cable installation barge (Bomel 

Ltd., pers. comm.); 

Cable installation vessels will always display cable laying signals to warn 

passing traffic. Similarly, radio navigation warnings will be requested and 

broadcast in addition to advance notification of works in a notice to Mariners 

and Kingfisher bulletins and Kingfisher Information charts; and 

Cable layers are typically supported by various vessels including anchor 

handlers, dive support vessels, ROV vessels and personnel launches. The 

activity of such vessels should be under the control of the operations 

superintendent of the main installation vessel to ensure coordination. 

The effects on shipping and navigation will be described within the project’s 

navigation risk report, and possible risk mitigation measures suggested. As part 

of the project’s required Navigational Risk Assessment, the DTI (now BERR) 

guidance (2005a) advocates implementation of a proposed Marine Navigational 

Safety Goal which should be managed through the life of the offshore installation. 

This includes a list of measures that serve to reduce risk to that which is “as low 

as reasonably practical” and that ensures “relevant good practice risk controls 

are in place”. 

The development of Emergency Response Plans for each site is also recommended 

as part of the guidance such that the lines of responsibility and reporting are 

clear and predefined, to minimise any potential risks to human life and the 

environment (DTI, 2005a). 


Vessels associated with cable installation works may lead to some temporary 

minor disruption to regular traffic and navigation; however the degree of 

these effects will be site specific and can be minimised through planning and 

liaison with appropriate regulators and other sea users. Best practice mitigation


measures (as also cited above) relevant to construction and maintenance activity 

include: 

● 

● 

● 

● 

● 

● 

● 

Circulation of information to vessel operations prior to operations; 

General notices via Navtex, Notices to Mariners and Admiralty Charts; 

Workshops to discuss navigational issues during construction; 

Use of guard vessels during construction; 

Monitoring of shipping during construction; 

Development of Emergency Response Plans; and 

Measures to advocated by BERR’s proposed Marine Navigational Safety 

Goal. 

Other guidance for ship collision avoidance also exists (UKOOA, 2003) and 

good practice guidelines should be followed whenever it is intended to anchor 

or locate a vessel within two kilometres of cables, pipelines and other subsea 

installations (UKOOA, 2002). 

It should be noted that Safety Zones as defined under the Energy Act 2004, which 

serve to protect safety of life around offshore installations, only apply to the 

offshore wind farm structures and not the associated vessels. Exclusion areas 

around installation vessels cannot therefore be enforced under these provisions. 

Guidance in ICPC (1996), cautions fishermen to keep at least one nautical mile 

away from a cable laying vessel and that fishing gear should never be operated 

astern of such a vessel for risk of engaging with a plough which will typically 

be operating three times the water depth away from the stern of the main cable 

installation vessel. 

5.9 Seascape and Visual Character 

BERR has produced guidance on the assessment of seascape and visual impacts 

of offshore wind farms (DTI, 2005b) This guidance makes recommendations on 

how to assess and deal with the seascape and visual impact assessment element 

of an EIA for an offshore wind farm development. 

Seascape and visual impacts related to the cable installation activities and not 

considered to be potentially significant, will be limited to: 

● 

● 

● 

Presence of the cable installation vessel and support vessels; 

Associated activity including plant and people present in the intertidal area; 


Possible sea surface aesthetic effects from any sediment plume generated as 

part of the installation, particularly where chalk is disturbed. 

133



5.9.1 POTENTIAL EFFECTS 

Cable installation activities will have a physical impact on the environment (see 

Section 4) and may, therefore, alter its visual appearance. The key issues of 

concern are the effects upon unspoilt landscapes and seascapes, designated and 

valued landscapes/seascapes and effects on visual amenity. 

The severity of the visual and landscape impacts arising from cable installation 

activities will be related to the value and use of the area and the scale of the 

alteration of the appearance. There will be trenching or ploughing activities 

taking place in the intertidal zone and adjacent coastal area necessary for the 

burial of the cable, which will be visible. However, the impacts to the landscape 

and seascape will be localised and short term. 

There will be landscape and visual impacts as a result of the presence of electricity 

substations and overhead transmission lines and poles connecting the offshore 

wind farm to the local electricity grid on land, and temporary disturbance of 

the seascape arising from the presence of subsea cable burial machinery and 

vessels. 


Visual and seascape impacts can only be effectively mitigated at the landfall 

site and will include usual good construction practice, such as limiting the area 

to be disturbed; maintaining tidy and compact site compounds and ensuring 

full restoration of the site in consultation with the relevant authorities (Natural 

England, Environment Agency or the Local Planning Authority). 

In addition, potential effects of cabling activities in the intertidal zone and 

adjacent coastal area may be minimised by timing trenching activities to avoid 

sensitive periods such as busy tourist seasons. 

5.10 Marine and Coastal Archaeology 

The potential archaeological resource that may be impacted by cable installation 

activities includes submerged palaeo-landscapes, including evidence of former 

human habitation or climate change; wrecks and related maritime remains and 

terrestrial archaeology, including in situ sites and / or findspots. 

Potentially significant effects include: 

● 

● 

134 

Direct loss or disturbance by cable installation activities; and 

Indirect disturbance via changes in sedimentation, such as increased erosion 

or accretion. 

The protection of archaeological, cultural heritage and wrecks are provided by a 

number of Legislative Acts. The principal protection for underwater heritage in


the UK’s territorial waters is provided by the Protection of Wrecks Act 1983. More 

recently, the Natural Heritage Act (2002) has enabled English Heritage to assume 

responsibility for maritime archaeology in English coastal waters. Specifically 

the Joint Nautical Archaeology Policy Committee (JNAPC) brings together a 

wide range of organisations with a direct and active interest and expertise in the 

marine historic environment. 

Relevant guidance provided by English Heritage and others includes: 

● 

● 

● 

● 

● 

England’s Coastal Heritage; 

Identifying and Protecting Palaeolithic Remains: Archaeological Guidance for 

Planning Authorities and Developers; 

Military Aircraft Crash Sites: archaeological guidance on their significance 

and future management; 

Joint Nautical Archaeology Policy Committee, 2006, Maritime Cultural 

Heritage and Seabed Development JNAPC Code of Practice for Seabed 

Development; and 

Wessex Archaeology (in consultation, 2006) Historic Environment Guidance 

Note for the Offshore Renewable Energy Sector. 


Direct loss or disturbance 

Cable installation activities including the anchoring of vessels can disturb the 

seabed in ways which could damage or destroy historic artefacts and submerged 

archaeological sites and features. In addition, indirect disturbance may occur 

from the longer term changes in the scouring and sedimentation patterns arising 

from cable installation and cable protection methods, exposing previously 

buried sites to degradation, destabilisation and corrosion. 

Archaeological remains and artefacts are a finite and non-renewable resource 

and, as such, cannot be replaced or recover from damage caused to physical 

properties or archaeological context. All direct impacts upon archaeology would 

be permanent and, therefore, of significance. However, the scale of significance 

would depend upon the strength of the impact ranging from low, if the material 

is simply dislodged from its resting place, to high, if the material is crushed or 

damaged by the installation equipment. 

Unlike aggregate extraction where the sediment is brought to the surface 

increasing the likelihood of identifying archaeological finds, the excavated 

sediment from cable burial machines is not brought to the surface. The sediment 

is ploughed or jetted to the side of the trench and thus there is no opportunity, 

at present, to investigate the occurrence of archaeological remains during cable 

installation activities, unless they are identified by the sonar systems, cable 

135



tone trackers or subsea video systems equipment housed on certain installation 

devices. 

The significance of impacts on artefacts and submerged archaeological sites 

will be similar for all cable installation devices. If artefacts or wrecks lie on or 

just beneath the seabed surface, they will be displaced from their resting place, 

crushed or compacted by the cabling equipment (i.e. skids or tracks). In contrast, 

the significance of the impact on artefacts of wrecks situated on or just beneath 

the seabed surface will vary with the type of burial tool used. Rock cutting and 

chain excavating tools will have a more significant impact which rip, slice and 

scrape at the substrata, compared with jetting tools which liquefy or erode 

sediments. The impact will be less in cohensionless substrata where relatively 

low pressures are required to liquefy the sediments, compared with cohesive 

substrata whereby localised erosion and scouring of the sediments occurs. 

Indirect disturbance via changes in sedimentation 

During the operational phase, there is the potential for exposure of archaeological 

material through scour along the cable route. The scale of the impact will be site 

specific and depend on local hydrology and geology. 


To ensure that comprehensive treatment is afforded to historic environment 

interests, it is recommended that early dialogue is initiated with English Heritage 

for the subtidal cable route and with the local authority (i.e. County Council) 

historic environment service for the intertidal and terrestrial sections of the cable 

route. Early negotiation will assist in the evaluation of any necessary mitigation 

as the project develops. 

The basic principle with regard to any known or potential archaeological feature 

or site is one of avoidance. Where possible, within technical constraints, cable 

routes should be grouped together to reduce the area and minimise the impacts. 

Where avoidance is not possible, the archaeological/historical site should be 

investigated prior to cabling activities to determine its importance and any suitable 

mitigation measures necessary. To effectively mitigate any potential impacts on 

known archaeological sites and important land and seascapes within a study, 

all aspects of any archaeological work will be detailed by a Written Scheme of 

Investigation (WSI). This provides for all forms of archaeological mitigation that 

may be required in light of pre and post-installation investigations, including 

archiving and dissemination of results. It is usually then subject to the approval 

of the County Archaeologist and English Heritage. FEPA conditions usually 

stipulate that no works are to take place until a protocol has been submitted to 

the Licensing Authority which has been formally agreed with an Archaeologist 

representing the County Council adjacent to the site of work. The protocol must 

detail what action is to be taken to protect any archaeological and shipwreck 

136


remains identified in the Environmental Statement submitted in support of the 

applications for consent for the works (Cefas, pers. comm.). 

Construction Exclusion Zones can also be used where known or potential 

archaeological sites or geophysical anomaly are located within the cable route 

and buffer zones. The size of the exclusion zone size usually depends upon the 

extent of the known or suspected archaeology, although they may be subject to 

movement, reduction or removal following further survey work prior to cabling 

activities. If cable routes cannot be altered to avoid sites of high importance, 

then subsequent evaluation may result via excavation or recording in situ. 

Within the intertidal zone and adjacent terrestrial environment, close contact 

should be maintained with English Heritage and the local authority archaeological 

services with respect to any archaeological material that may be encountered. 

A watching brief may be necessary if trench excavation is proposed within the 

foreshore or adjacent terrestrial area. 

137

6 Good practice measures 

This section provides examples of good practice measures which could be 

adopted during all phases of project planning. Such measures could be used 

in conjunction with mitigation measures defined in Section 5.0 to minimise the 

magnitude and significance of effects to the local environment. 

Examples of good practice measures which could be adopted to reduce potential 

disturbance of cabling activities on intertidal and subtidal habitats, marine 

mammals, birds, fish and shellfish include: 

● 

● 

● 

● 

● 

● 

● 

138 

Early dialogue with the appropriate regulatory and advisory authorities 

(Department for Transport, Department for Environment Food and Rural 

Affairs, Centre for Environment, Fisheries and Aquaculture Science, Natural 

England, Countryside Council for Wales, Scottish Natural Heritage, the 

Environment and Heritage Service Northern Ireland, the Environment Agency 

and the Joint Nature Conservation Committee). 

Sensitive timing and routing of cable installation to avoid important feeding, 

breeding/spawning and nursery areas and seal haul out areas especially 

during sensitive periods (breeding season); 

Avoidance of areas of sensitive habitat such as biogenic reef. 

Sensitive timing and routing of maintenance vessels to reduce number of 

trips; 

For marine mammals and birds: preparation of on-site protocol in sensitive 

locations; 

For marine mammals and birds: briefing of cable installation contractor’s 

personnel for on-site procedures and protocol; and 

Monitoring effects using a Before-After-Control-Impact (BACI) study. 


disturbance of cable installation activities on shipping and navigation include: 

● 

● 

● 

● 

Early dialogue with the Maritime & Coastguard Agency (MCA), Hydrographer 

of the Navy and local harbour authorities should be undertaken in relation to 

shipping and navigational issues; 

Planning, liaison and consultation with local experts and users (harbour 

masters, coastguard etc) to identify impacts to traffic and navigation; 

Production of Emergency Response Plans to minimise potential risks to 

human life and the environment; and 

Following good practice guidelines for ship collision avoidance and anchorage 

near cables, installations etc. (UKOOA, 1997, 2002, 2003).

Good Practice Measures 


disturbance of cable installation activities on archaeology include: 

● 

● 

Early dialogue with English Heritage / Welsh Historic Buildings (CADW), 

Historic Scotland / Environment and Heritage Service (EHS) Northern Ireland 

and with the local authority (i.e. County Council) to discuss the potential 

impacts and mitigation measures of cabling activities on archaeological and 

historic issues; and 

Investigation of cable survey route in terms of potential archaeological / 

historical sites. 

139

7 Gaps in understanding 

Research for this report has highlighted a number of gaps in available data 

and in understanding of the actual impacts resulting from cable burial activities 

associated with the offshore wind farm industry. The general lack of understanding 

is due to the physical and biological effects of cable burial being very site 

specific, particularly with regard to sites with different sediment characteristics. 

What makes this even more difficult to interpret is that the majority of sites will 

experience a large variation of sediment types along the cable routes. There has 

also been a deficit in the monitoring of cable burial activities in the past as the 

impacts are regarded as secondary in terms of scale when compared with those 

from the installation and operation of the wind turbines. 

There is limited centralised knowledge sharing both with the offshore wind farm 

industry and from other more established marine cabling industries such as 

subsea telecommunications and the oil and gas sector. Information that does 

exist is not widely disseminated, reviewed or synthesised for a wider audience 

or made publicly available. 

There is limited documentation and research carried out to date on the 

quantification of material disturbed and brought into suspension from cable 

burial operations, in particular, from burial methods using jetting, cutting, 

dredging and excavating tools. This lack of basic information on the volumes of 

sediment brought into suspension limits the potential of modelling techniques 

and establishing the fate of sediment plumes. Research from other industries 

can be used for predicting potential effects but the scale of impacts is quite 

different (as identified in Section 4.4) and the effect will depend on the sensitivity 

of the receptor and the site conditions. Should the generation of a sediment 

plume be considered to be an area of concern, in situ monitoring of a number 

of different techniques and tools in one location, together with further plume 

modelling would be required in order to draw comparisons under the same site 

specific conditions. Confidence in the modelling could be attained by comparing 

predicted versus in situ results. The sediment monitoring requirements included 

in FEPA licences for UK Round 1 and potentially Round 2 offshore wind farm 

applications will also provide valuable data for the quantification of the impact 

of cable burial operations. 

In addition, there is a general lack of understanding of the biological response 

to sediment plumes, arising from any activities that disturb the seabed, such 

as the sensitivity and tolerance of species and habitats to different levels of 

sediment plumes and different plume durations. Additional research in this area 

is therefore necessary in order to further this knowledge base to enable accurate 

predictions of sensitivity to be determined. There are, however difficulties in 

undertaking research to fill these gaps such as the problem associated with 

site specific differences changing the scale of the effects and the adaptability of 

species in different environments and at different times of the year. Targeted 

research involving the testing of different cable burial devices and tools in the 

same conditions (seabed types, wave and tidal conditions) could overcome 

140

Gaps in understanding 

part of this problem but the different levels of adaptability of species will need 

to be assessed on each occasion. This more generalised research requirement 

is much wider ranging than is necessary for cable burial alone as it involves a 

number of industries with an interest in the marine environment. 

The need for further studies and monitoring to investigate the potential impacts 

needs to be put into context with the amount of sediment that is put into 

suspension during cabling in comparison to other activities (i.e. certain fishing 

techniques and dredging activity) and natural events (i.e. severe storms). 

141

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153

Appendix A: Standards and 

codes of practice relevant 

to cable installation & the 

offshore wind industry 

General 

The standards set out below should be regarded as defining the minimum 

required for subsea cables associated with an offshore wind farm. 

Minimum Requirements 

Works should comply with all appropriate statutory acts and regulations, 

including, but not limited to, the latest revisions of the: 

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154 

Health & Safety at Work Act 1974; 

Management of Health & Safety at Work Regulations 1999; 

Health & Safety (First Aid) Regulations 1981; 

The Reporting of Injuries Diseases & Dangerous Occurrences Regulations 

1995; 

The Workplace (Health, Safety & Welfare) Regulations 1992; 

Personal Protective Equipment at Work Regulations 1992; 

The Manual Handling Operations Regulations 1992; 

The Provision & Use of Work Equipment Regulations 1998; 

The Lifting Operations & Lifting Equipment Regulations 1998; 

Health & Safety (Display Screen Equipment) Regulations 1992; 

Control of substances Hazardous to Health Regulations 1999; 

The Noise at Work Regulations 1988; 

Electricity at Work Regulations 1989; 

The Merchant Shipping Act 1988; 

Construction (Design & Management) Regulations 1994; 

Construction (Health, Safety & Welfare) Regulations 1996; 

Confined Space Regulations 1997; 

Fire Precautions (Workplace) Regulations 1997;

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The Fire Precautions Act, 1971; 

The Manual Handling Operations Regulations, 1992; 

Appendix Tables 

The Transport of Dangerous Goods (Classification Packaging and Labelling) 

and use of Transportable Pressure Receptacles Regulations, 1996; 

The Transport of Dangerous Goods (Safety Advisors) Regulations, 1999; 

The Electrical Equipment (Safety) Regulations, 1994; 

The Construction (Head Protection) Regulations, 1989; 

The Pressure Systems Safety Regulations, 2000; 

The Pressure Equipment Regulations, 1999; 

The Ionising Radiations Regulations, 1999; 

The Control of Asbestos at Work Regulations, 1998; 

The Control of Lead at Work Regulations, 1998; 

The Highly Flammable liquids and Liquefied Petroleum Gases Regulations, 

1972; 

The Petroleum-Spirit (Plastic Container) Regulations, 1989; 

The Safety Representatives and Safety Committees Regulations, 1977; 

The Electricity Act, 1989; 

The Health and Safety (Consultation with Employees) Regulations, 1989; 

The Health and Safety Information for Employees Regulations, 1989; 

The Health and Safety (Safety Signs and Signals) Regulations, 1996; 

The Health and Safety (Enforcing Authority) Regulations, 1998; 

The Health and Safety (Training for Employment) Regulations, 1990; 

The Working Time Regulations, 1998; 

The Control of Major Accident Hazard Regulations, 1999; 

Electricity Safety, Quality and Continuity Regulations 2002; 

Marine Guidance Note MGN 275(M); 

The Grid Code, Issue 3, Revision 13, January 2006; 

Guidance Notes for Power Park Developers, Grid Code Connection Conditions 

Compliance: Testing & Submission of the Compliance Report, June 2005 – 

Issue 1; 

The National Grid and EDF Connection Offers, in particular Appendix F of the 

NG offer, a copy of which is provided; 

The Distribution Code and the Code to the Distribution Code of Licensed 

Distribution Network Operators of GB – Issue 05, August 2004; 

155



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156 

The Distribution Safety Rules of EDF Energy Networks; and 

GB Security and Quality of Supply Standard. 

Unless noted to the contrary, all equipment, materials and labour should be 

supplied, designed and constructed in accordance with the applicable sections 

of the latest revisions of the following: 

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● 

● 

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British Standards (BS); 

ISO standards (ISO); 

DNV standards (DNV); 

Electricity Networks Association standards (ENA) – applicable to possible 

DNO future owned equipment; and 

International Electro-technical Commission standards (IEC). 

Electrical Supply Characteristics and Conditions 

Aside from the Grid Code, Distribution Code and Connection Agreement 

conditions the latest editions of the following should be applicable to the 

electrical connection in line with standard UK practices: 

● 

● 

● 

● 

● 

● 

● 

Engineering Recommendation P28 – Planning Limits for Voltage Fluctuations 

Caused by Industrial, Commercial and Domestic Equipment in the United 

Kingdom; 

Engineering Recommendation P29 – Planning Limits for Voltage Unbalance 

in the United Kingdom; 

Engineering Recommendation G5/4-1 – Planning Levels for Harmonic Voltage 

Distortion and the Connection of Non-Linear Equipment to Transmission 

Systems and Distribution Networks in the United Kingdom; 

Engineering Recommendation G59/1 – Recommendations for the Connection 

of Embedded Generating Plant to the Regional Electricity Companies’ 

Distribution Systems; 

Engineering Recommendation G75 – Recommendations for Embedded 

Generating Plant Connecting To Public Electricity Suppliers’ Distribution 

Systems Above 20kV or with Outputs Over 5MW; 

ENA Technical Specification 41-24 – Guidelines for the Design, Installation, 

Testing and Maintenance of Main Earthing Systems in Substations; and 

Engineering Recommendation S34 – A Guide for Assessing the Rise of 

Potential at Substation Sites.

Power Cables 


The latest editions of the following standards should be considered where 

appropriate along with other relevant international or national standards as 

consistent with good UK engineering practices: 

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● 

● 

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IEC 60228 – Conductors of insulated cables; 

IEC502 – Extruded solid dielectric insulated power cables for rated voltages 

from 1kV up to 30kV; 

IEC60840 Tests for power cables with extruded insulation for rated voltages 

above 30kV (Um = 36kV) up to 150kV (Um = 170kV); 

IEC 60885-3 – Partial discharge tests; 

IEC 287 – Calculation of continuous current rating of power cables 

(100% load factor); 

IEC 853 – Calculation of cyclic and emergency current rating of cables; 

BS 1441 – Galvanised steel wire for submarine cables; 

ENATS 09-16 – Tests on power cables with XLPE insulation and metallic 

sheath and their accessories, for rated voltages of 66kV (Um = 72.5kV), 110kV 

(Um = 123kV) and 132kV (Um = 145kV); 

CEGB – TDM 99/45 and 99/56; 

ICEA S68 516 – Ethylene propylene rubber insulated wire and cable for the 

transmission and distribution of electrical energy; 

AEIC CS6-87 or 96 – Specifications for ethylene propylene rubber insulated 

shielded power cables rated 5kV through 69kV; 

BS 6469 – Insulating and sheath materials of electric cables; 

ER 55/4 – Bonding systems; and 

ERA Report F/T 186 – Power cable ratings. 

Optical Fibre Cables 

The latest editions of the following standards should be considered where 

appropriate along with other relevant international or national standards as 

consistent with good UK engineering practices: 

● 

● 

● 

● 

● 

IEC 793-1 Optical fibres part 1: Generic specification; 

IEC 793-2 Optical fibres part 2: Product specification; 

IEC 794-1 Optical fibre cables part 1: Generic specification, measurements 

and tests; 

IEC 794-2 Optical fibre cables part 2: Product specification; 

IEC 1073 -1 Splices for optical fibres and cables part 1: Generic specification; 

157



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158 

IEC 1073-2 Splices for optical fibres and cables part 2: Splice organisers and 

closures for optical fibre cables; 

ITU-T G.651 Characteristics of a 50/125 micro metre multimode graded index 

optical fibre cable; 

ITU-T G.652 Characteristics of a single mode optical fibre cable; 

ITU-T G.653 Characteristics of a dispersion-shifted single-mode optical fibre 

and cable; 

ITU-T G.654 Characteristics of a cut-off shifted single-mode optical fibre and 

cable; 

ITU-T G.655 Characteristics of a non-zero dispersion-shifted single-mode 

optical fibre and cable; 

ITU-T G.656 Characteristics of a fibre and cable with non-zero dispersion for 

wideband transport; 

ITU-T G.664 Optical safety; 

ITU-T L13.3 Sheath joints and organisers of optical fibre cables in the outside 

plant; 

BS EN 60793 Optical fibres – testing; 

BS EN 60811 Insulation and sheathing materials for cables; 

BS EN 61663-1 Lightening protection – fibre optic installations; 

BS EN 60874-10-3 Connectors for fibres and cables – BFOC/2.5 adaptor; 

BS EN 61073 Mechanical splices and fusion splices for optical cables; 

BS EN 187000 Generic specification for optical fibre cables; 

BS EN 187101 Optical telecommunication cables to be used in ducts or direct 

buried application; 

BS EN 188000 Generic specification for optical fibres; 

BS EN 188100 Sectional specification: single-mode (SM) optical fibre; and 

BS EN 188101 Family specification: single-mode dispersion unshifted optical 

fibre. 

Electricity Supply Industry Requirements 

If the project is a Round Two proposal, it will require BERR consents and 

licensing. Under the generation licence (which is required for generating plant 

over 100MW) there will be a requirement to become party to a number of core 

industry agreements. The developer will need to meet the following consents 

and licensing requirements: 

● 

A generation licence (via Ofgem and granted by Secretary of State).

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● 


Under the British Trading and Transmission Arrangements (BETTA), 

generation licensees shall be required to become party to the Balancing and 

Settlement Code (BSC) to include completion of systems and procedures 

testing processes; 

Generation licensees are required to be party to The Grid Code, Issue 3, 

Revision 13, January 2006 – National Grid, which defines the operating 

procedures and principles governing the National Grid’s relationship with all 

users of the GB Transmission System, and the procedures for planning and 

operation; 

Generation licensees are required to adhere to the GB Distribution Code; 

Generation licensees are required to be party to the Connection and Use of 

System Code (CUSC); 

In addition, the developer shall be required to comply with the technical 

requirements on a site specific basis with the local Distribution Network 

Operator (DNO) and National Grid. These shall include, but not be limited to 

Appendix F of the National Grid Connection Offer; and 

Wayleaves may be required for new electrical lines and infrastructure and 

contain specific requirements. 

Other Codes 

Work should also fully comply with the requirements of: Maritime and Coastguard 

Agency – Marine Guidance Note MGN 275 (M). 

159

Printed in UK on recycled paper. 

Department for Business, Enterprise and Regulatory Reform. www.berr.gov.uk 

First published January 2008. © Crown Copyright. URN 07/1680.

Review of Cabling Techniques and Environmental Effects Applicable

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